feat: add Mantle override for AWS Bedrock Anthropic endpoint

Introduce `model_capabilities.mantle` flag that installs a provider-specific
override in fast-agent's `ModelDatabase._PROVIDER_MODEL_OVERRIDES` to strip
features the AWS Bedrock Mantle endpoint rejects (beta headers, extended
thinking, task budgets, web tools, prompt caching).

Without this override, fast-agent sends default beta headers and `thinking`
parameters for modern Claude models that Mantle rejects with a misleading
404 "model does not exist" error.

README.md

@@ -89,11 +89,32 @@ model_capabilities:
   vision: false
   context_window: 200000
   max_output_tokens: 32000
+  mantle: false               # optional — see "Mantle override" below
 ```
 
 Capabilities are published in the registry and used to register unknown models
 with fast-agent's `ModelDatabase`.
 
+### Mantle override (`model_capabilities.mantle: true`)
+
+Set this when the `anthropic.base_url` points at the AWS Bedrock **Mantle**
+endpoint (`https://bedrock-mantle.{region}.api.aws/anthropic`). Pallas then
+installs a provider-specific override for `(Provider.ANTHROPIC, model_name)`
+in fast-agent's `ModelDatabase._PROVIDER_MODEL_OVERRIDES` that clones the
+model's base parameters but strips the features Mantle rejects:
+
+- `anthropic_required_betas` — no `anthropic-beta: ...` header
+- `reasoning` / `reasoning_effort_spec` — no extended-thinking request
+- `anthropic_task_budget_supported` — no task budget
+- `anthropic_web_fetch_version` / `anthropic_web_search_version` — no web tools
+- `cache_ttl` — prompt caching disabled
+
+Without this flag, fast-agent sends its default beta headers and `thinking`
+parameters for modern Claude models (e.g. Opus 4.7, Sonnet 4.6) which Mantle
+rejects with a misleading `404 "The model '...' does not exist"`. See
+`docs/bedrock.md` for the full configuration walkthrough.
+
+
 ---
 
 ## Environment variable