Fixes #5173 — gpt-5.4 shows 32k context in Hermes instead of 1,050,000

Root Cause

Two independent bugs conspire to produce the wrong context window:

1. Missing DEFAULT_CONTEXT_LENGTHS entries — gpt-5.4 (and other gpt-5.x variants) were absent from the fallback dict. Lookups fell through to the generic "gpt-5": 128000 catch-all, returning 128k instead of 1,050,000.

2. Cache poisoning — When connecting via the Codex endpoint, Hermes probes the API and may receive max_output_tokens (32k) where it expects context_length. That value gets written to context_length_cache.yaml. Since the persistent cache is checked first in the resolution order, the bad 32k value overrides everything permanently.

Fix (two-part)

1. Add specific gpt-5.x entries to DEFAULT_CONTEXT_LENGTHS

New entries added before the generic "gpt-5": 128000 catch-all in agent/model_metadata.py:

The existing sorted() in get_model_context_length ensures longest-key-first matching, so specific variants correctly shadow the catch-all.

2. Sanity guard in save_context_length()

Added a pre-write check: if the model name contains "gpt-5" and the value being cached is <= 128,000, the write is rejected and a warning is logged. This stops max_output_tokens (32k) from ever being written into context_length_cache.yaml for gpt-5 family models.

The guard does not affect non-gpt-5 models — e.g. llama-3 can still be cached at 32k normally.

Users who need to force a specific value can always set model.context_length in config.yaml, which is checked before the cache.

Tests

Added TestGpt5ContextLengths in tests/agent/test_model_metadata.py:

• gpt-5.4 -> 1,050,000 via DEFAULT_CONTEXT_LENGTHS
• gpt-5.4-mini -> 1,050,000 via DEFAULT_CONTEXT_LENGTHS
• save_context_length("gpt-5.4", ..., 32000) -> silently rejected
• save_context_length("gpt-5.4", ..., 1_050_000) -> cached successfully
• Sanity guard does NOT block llama-3 at 32k

All 80 tests pass.