fix(anthropic): restrict fast mode to Opus 4.6 (Anthropic API contract)

Per https://platform.claude.com/docs/en/build-with-claude/fast-mode:
"Fast mode is currently supported on Opus 4.6 only. Sending speed: fast
with an unsupported model returns an error."

Pre-fix, _is_anthropic_fast_model() returned True for any claude-* model,
so /fast on Opus 4.7 (or Sonnet/Haiku) would persist agent.service_tier=fast
in config.yaml and the adapter would inject extra_body["speed"] = "fast"
on every subsequent request. Opus 4.7 returns:

  HTTP 400: 'claude-opus-4-7' does not support the `speed` parameter.

This wedged sessions across model upgrades (a user who ran /fast on Opus 4.6
and later switched the default model to 4.7 hit a hard 400 on every turn
until they manually edited config.yaml).

Changes:
- _is_anthropic_fast_model: gate on "opus-4-6" / "opus-4.6" only
- anthropic_adapter: add _supports_fast_mode predicate as defensive guard
  so stale request_overrides on an unsupported model are dropped silently
  instead of 400'ing
- Tests: flip the assertions that mirrored the bug (Sonnet/Haiku/Opus 4.7
  asserting fast-mode support) to match the documented API contract

hermes_cli/models.py

@@ -1740,10 +1740,20 @@ def model_supports_fast_mode(model_id: Optional[str]) -> bool:
 
 
 def _is_anthropic_fast_model(model_id: Optional[str]) -> bool:
-    """Return True if the model is a Claude model eligible for Anthropic Fast Mode."""
+    """Return True if the model is a Claude model eligible for Anthropic Fast Mode.
+
+    Fast mode is currently supported on Claude Opus 4.6 only. Per Anthropic's
+    docs (https://platform.claude.com/docs/en/build-with-claude/fast-mode):
+    "Fast mode is currently supported on Opus 4.6 only. Sending speed: fast
+    with an unsupported model returns an error." Opus 4.7 explicitly rejects
+    the ``speed`` parameter with HTTP 400.
+    """
     raw = _strip_vendor_prefix(str(model_id or ""))
     base = raw.split(":")[0]
-    return base.startswith("claude-")
+    if not base.startswith("claude-"):
+        return False
+    # Only Opus 4.6 supports fast mode at present.
+    return "opus-4-6" in base or "opus-4.6" in base
 
 
 def resolve_fast_mode_overrides(model_id: Optional[str]) -> dict[str, Any] | None: