Extract memory injection mode policy (#1986)

## Description

Extracts memory-injection mode resolution from `helpers.py` into
`headroom.proxy.memory_injection_mode_policy`. The proxy still reads
`HEADROOM_MEMORY_INJECTION_MODE` at request time, while the allowed
values/default/error contract is now pure and directly tested.

Closes #

## Type of Change

- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [x] Code refactoring (no functional changes)

## Changes Made

- Added `memory_injection_mode_policy.py` with the allowed mode type,
env name/default, and resolver.
- Kept `helpers.get_memory_injection_mode` as the request-time env
reader and compatibility entry point.
- Added direct policy tests for defaults, accepted values,
normalization, and invalid mode rejection.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
python -m pytest tests\test_memory_injection_mode_policy.py tests\test_proxy_system_prompt_immutable.py
10 passed in 14.60s

python -m ruff check .
All checks passed!

python -m ruff format --check .
1069 files already formatted

python -m mypy headroom --ignore-missing-imports
Success: no issues found in 410 source files

gitleaks protect --staged --no-banner --redact
no leaks found
```

## Real Behavior Proof

- Environment: Windows, Python 3.13.13, branch
`jd/architecture-slice-33`.
- Exact command / steps: ran new memory injection mode policy tests,
existing system-prompt immutability tests, ruff, ruff format check,
mypy, and staged gitleaks scan.
- Observed result: memory injection mode behavior remains covered and
local lint/type/security checks pass.
- Not tested: live proxy request; existing helper entry point remains
intact.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

N/A

## Additional Notes

Documentation and changelog updates are N/A for this internal
architecture-only refactor. The push reported existing default-branch
Dependabot alerts; no staged secret leaks were found for this PR.

---------

Co-authored-by: Tejas Chopra <chopratejas@gmail.com>

headroom/proxy/helpers.py

@@ -26,6 +26,7 @@ from headroom import paths as _paths
 from headroom._subprocess import run
 from headroom.proxy import (
     diagnostic_decode_policy,
+    memory_injection_mode_policy,
     query_log_policy,
     request_limit_policy,
     sse_byte_buffer_policy,
@@ -232,9 +233,9 @@ def capture_codex_wire_debug(
 # Configurable via HEADROOM_MEMORY_INJECTION_MODE env var. There is no
 # "system_prompt" option — that path is permanently retired by I2 (cache hot
 # zone never modified). See REALIGNMENT/02-architecture.md §2.2.
-_MEMORY_INJECTION_MODE_ENV = "HEADROOM_MEMORY_INJECTION_MODE"
-_MEMORY_INJECTION_MODE_DEFAULT: Literal["live_zone_tail", "disabled"] = "live_zone_tail"
-MemoryInjectionMode = Literal["live_zone_tail", "disabled"]
+_MEMORY_INJECTION_MODE_ENV = memory_injection_mode_policy.MEMORY_INJECTION_MODE_ENV
+_MEMORY_INJECTION_MODE_DEFAULT = memory_injection_mode_policy.MEMORY_INJECTION_MODE_DEFAULT
+MemoryInjectionMode = memory_injection_mode_policy.MemoryInjectionMode
 
 
 def get_memory_injection_mode() -> MemoryInjectionMode:
@@ -243,13 +244,8 @@ def get_memory_injection_mode() -> MemoryInjectionMode:
     Read at request time so the env var can be flipped without restart for
     smoke tests. Unknown values are rejected loudly (no silent fallback).
     """
-    raw = os.environ.get(_MEMORY_INJECTION_MODE_ENV, "").strip().lower()
-    if not raw:
-        return _MEMORY_INJECTION_MODE_DEFAULT
-    if raw in ("live_zone_tail", "disabled"):
-        return cast(MemoryInjectionMode, raw)
-    raise ValueError(
-        f"Invalid {_MEMORY_INJECTION_MODE_ENV}={raw!r}; expected 'live_zone_tail' or 'disabled'"
+    return memory_injection_mode_policy.resolve_memory_injection_mode(
+        os.environ.get(_MEMORY_INJECTION_MODE_ENV)
     )
 
 

headroom/proxy/memory_injection_mode_policy.py

@@ -0,0 +1,22 @@
+"""Memory-injection mode resolution policy."""
+
+from __future__ import annotations
+
+from typing import Literal, cast
+
+MEMORY_INJECTION_MODE_ENV = "HEADROOM_MEMORY_INJECTION_MODE"
+MEMORY_INJECTION_MODE_DEFAULT: Literal["live_zone_tail", "disabled"] = "live_zone_tail"
+MemoryInjectionMode = Literal["live_zone_tail", "disabled"]
+
+
+def resolve_memory_injection_mode(raw: str | None) -> MemoryInjectionMode:
+    """Resolve the active memory-injection routing mode from an optional value."""
+    normalized = (raw or "").strip().lower()
+    if not normalized:
+        return MEMORY_INJECTION_MODE_DEFAULT
+    if normalized in ("live_zone_tail", "disabled"):
+        return cast(MemoryInjectionMode, normalized)
+    raise ValueError(
+        f"Invalid {MEMORY_INJECTION_MODE_ENV}={normalized!r}; "
+        "expected 'live_zone_tail' or 'disabled'"
+    )

tests/test_memory_injection_mode_policy.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+import pytest
+
+from headroom.proxy.memory_injection_mode_policy import (
+    MEMORY_INJECTION_MODE_DEFAULT,
+    resolve_memory_injection_mode,
+)
+
+
+def test_resolve_memory_injection_mode_defaults_for_missing_value() -> None:
+    assert resolve_memory_injection_mode(None) == MEMORY_INJECTION_MODE_DEFAULT
+    assert resolve_memory_injection_mode("") == MEMORY_INJECTION_MODE_DEFAULT
+    assert resolve_memory_injection_mode("   ") == MEMORY_INJECTION_MODE_DEFAULT
+
+
+def test_resolve_memory_injection_mode_accepts_known_values_case_insensitively() -> None:
+    assert resolve_memory_injection_mode("live_zone_tail") == "live_zone_tail"
+    assert resolve_memory_injection_mode(" DISABLED ") == "disabled"
+
+
+def test_resolve_memory_injection_mode_rejects_unknown_values() -> None:
+    with pytest.raises(ValueError, match="HEADROOM_MEMORY_INJECTION_MODE"):
+        resolve_memory_injection_mode("system_prompt")