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

agent/anthropic_adapter.py

@@ -76,6 +76,7 @@ _ADAPTIVE_THINKING_SUBSTRINGS = ("4-6", "4.6", "4-7", "4.7")
 # Models where temperature/top_p/top_k return 400 if set to non-default values.
 # This is the Opus 4.7 contract; future 4.x+ models are expected to follow it.
 _NO_SAMPLING_PARAMS_SUBSTRINGS = ("4-7", "4.7")
+_FAST_MODE_SUPPORTED_SUBSTRINGS = ("opus-4-6", "opus-4.6")
 
 # ── Max output token limits per Anthropic model ───────────────────────
 # Source: Anthropic docs + Cline model catalog.  Anthropic's API requires
@@ -219,6 +220,17 @@ def _forbids_sampling_params(model: str) -> bool:
     return any(v in model for v in _NO_SAMPLING_PARAMS_SUBSTRINGS)
 
 
+def _supports_fast_mode(model: str) -> bool:
+    """Return True for models that support Anthropic Fast Mode (speed=fast).
+
+    Per Anthropic docs, fast mode is currently supported on Opus 4.6 only.
+    Sending ``speed: "fast"`` to any other Claude model (including Opus 4.7)
+    returns HTTP 400. This guard prevents silently 400'ing when stale config
+    or older callers leave fast mode enabled across a model upgrade.
+    """
+    return any(v in model for v in _FAST_MODE_SUPPORTED_SUBSTRINGS)
+
+
 # Beta headers for enhanced features (sent with ALL auth types).
 # As of Opus 4.7 (2026-04-16), the first two are GA on Claude 4.6+ — the
 # beta headers are still accepted (harmless no-op) but not required. Kept
@@ -1932,9 +1944,15 @@ def build_anthropic_kwargs(
 
     # ── Fast mode (Opus 4.6 only) ────────────────────────────────────
     # Adds extra_body.speed="fast" + the fast-mode beta header for ~2.5x
-    # output speed. Only for native Anthropic endpoints — third-party
-    # providers would reject the unknown beta header and speed parameter.
-    if fast_mode and not _is_third_party_anthropic_endpoint(base_url):
+    # output speed. Per Anthropic docs, fast mode is only supported on
+    # Opus 4.6 — Opus 4.7 and other models 400 on the speed parameter.
+    # Only for native Anthropic endpoints — third-party providers would
+    # reject the unknown beta header and speed parameter.
+    if (
+        fast_mode
+        and not _is_third_party_anthropic_endpoint(base_url)
+        and _supports_fast_mode(model)
+    ):
         kwargs.setdefault("extra_body", {})["speed"] = "fast"
         # Build extra_headers with ALL applicable betas (the per-request
         # extra_headers override the client-level anthropic-beta header).