feat(providers): add extra_body config for OpenAI-compatible endpoints

Add an `extra_body` field to `ProviderConfig` that merges arbitrary
key-value pairs into every OpenAI-compatible request body. This is the
escape hatch for provider-specific features that nanobot does not have
first-class fields for.

Real-world use cases this unblocks via config alone (no code changes):
- vLLM/TGI `chat_template_kwargs` (e.g. `enable_thinking: false`)
- vLLM guided decoding (`guided_json`, `guided_regex`)
- Local model sampling params (`repetition_penalty`, `top_k`, `min_p`)
- Any future provider-specific param without a new PR each time

The config extra_body is applied last via recursive deep-merge, so it
can extend or override provider-specific defaults (e.g. thinking
params) without clobbering sibling keys set by internal logic.

Changes:
- Add `extra_body: dict[str, Any] | None` to `ProviderConfig`
- Pass it through `factory.py` to `OpenAICompatProvider.__init__`
- Deep-merge into `_build_kwargs` after all internal extra_body entries
- Add `_deep_merge` helper (recursive dict merge, does not mutate inputs)
- 21 tests: deep-merge semantics, provider init, _build_kwargs
  integration, thinking coexistence, real-world patterns (guided_json,
  repetition_penalty), and schema validation

nanobot/providers/factory.py

@@ -69,6 +69,7 @@ def make_provider(config: Config) -> LLMProvider:
             default_model=model,
             extra_headers=p.extra_headers if p else None,
             spec=spec,
+            extra_body=p.extra_body if p else None,
         )
 
     defaults = config.agents.defaults