fix(rust): wire ICM compressor into Rust proxy on /v1/messages

Adds an opt-in compression interceptor that buffers Anthropic
/v1/messages requests, runs IntelligentContextManager over the
messages array, and forwards the (possibly trimmed) body upstream.
All other paths, methods, and content-types stay on the original
streaming passthrough — so existing operators see zero change.

Behaviour gates ALL must be true to buffer + compress:
  - --compression flag (or HEADROOM_PROXY_COMPRESSION=1)
  - method == POST
  - path == /v1/messages
  - Content-Type: application/json
  - ICM constructed successfully at startup

Falls through to streaming on any failure: parse, missing fields,
unknown model, body-too-large. Compression must never break a
request — that's the safety contract.

Model context windows come from a vendored LiteLLM snapshot at
crates/headroom-proxy/data/model_prices_and_context_window.json
parsed once into an OnceLock<HashMap>. Refresh via
scripts/refresh_model_limits.sh. Rationale documented inline:
hardcoded tables silently rot; LiteLLM is the canonical source
the entire LLM-tooling ecosystem relies on.

New tests:
  - 16 unit tests across compression::{anthropic, icm, model_limits}
  - 5 integration tests: off-passthrough, on-short-passthrough,
    on-oversized-trim, on-non-json-skip, on-non-llm-path-skip

Verification:
  - cargo test --workspace -> 884 passed, 0 failed
  - cargo clippy --workspace -- -D warnings -> clean
  - cargo fmt --check -> clean

scripts/refresh_model_limits.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+#
+# Refresh the vendored LiteLLM model_prices_and_context_window.json
+# used by `crates/headroom-proxy/src/compression/model_limits.rs`.
+#
+# We vendor the snapshot rather than fetching at build/runtime so the
+# proxy binary ships with no network dependency at startup. Operators
+# tracking new model releases run this script and commit the diff.
+#
+# Validation:
+#   1. JSON parses
+#   2. Contains a known-stable Claude model entry
+#   3. Contains a known-stable GPT model entry
+# These guard against accidentally vendoring an empty / malformed file.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+DEST="$REPO_ROOT/crates/headroom-proxy/data/model_prices_and_context_window.json"
+URL="https://raw.githubusercontent.com/BerriAI/litellm/main/model_prices_and_context_window.json"
+
+echo "Fetching $URL"
+TMP="$(mktemp)"
+trap 'rm -f "$TMP"' EXIT
+curl -fsSL "$URL" -o "$TMP"
+
+# Validate the snapshot before swapping it in.
+python3 -c "
+import json, sys
+with open('$TMP') as f:
+    data = json.load(f)
+if not isinstance(data, dict):
+    sys.exit('top-level not an object')
+if 'sample_spec' not in data:
+    sys.exit('missing sample_spec entry — schema may have changed')
+# Spot-check stable entries.
+required = ['claude-sonnet-4-5-20250929', 'gpt-4o-mini', 'gpt-4-turbo']
+missing = [k for k in required if k not in data]
+if missing:
+    sys.exit(f'missing required entries: {missing!r}')
+print(f'OK: {len(data)} entries, including {required}')
+"
+
+mv "$TMP" "$DEST"
+trap - EXIT
+
+echo "Updated $DEST"
+echo "Run 'cargo test -p headroom-proxy --lib compression::model_limits' to verify."