Add extra_headers for OpenAI-family LLM providers

[uusp]

Summary

Adds operator-defined extra_headers for OpenAI-family text providers
(openai chat-completions and openai_responses), plumbed from [llm]
config through the runtime (LlmRuntimeConfig) and selector
(ProviderConfig / ProviderBuildContext) into request headers.
Adapter-managed framing (Authorization / Content-Type / Accept)
always wins; the merge is case-insensitive.

Motivation (verified against a live gateway)

The opencode Zen/Go gateway requires x-opencode-session on /responses
and answers 400 MissingSessionID without it:

{"type":"error","error":{"type":"MissingSessionID","message":"Error from provider (Console Go): Request is missing x-opencode-session and cannot be routed efficiently."}}

OpenSquilla had no way to send custom headers — no CLI flag, no schema
field (headers exists only on MemoryEmbeddingRemoteConfig) — so any
model behind such a gateway was unusable through it. Verified live with
curl: same key + endpoint + model fails without the header and succeeds
with it. After this change (plus the header in config), opensquilla agent
completes against that gateway.

Usage

[llm]
provider = "openai_responses"
model = "muse-spark-1.3-contributor"
api_key_env = "GOKEY"
base_url = "https://opencode.ai/zen/go/v1"
extra_headers = { x-opencode-session = "my-session" }

Never store secrets in extra_headers; Authorization cannot be
overridden through it. CLI wizard / Web UI fields for this are intentionally
left out to keep this change small — happy to follow up if wanted.

Tests

• New tests/test_provider_extra_headers.py: 10 offline tests
  (MockTransport wire assertions for both adapters including list_models,
  reserved-header protection, selector plumbing, config parsing). 10/10 pass.
• ruff check clean on all touched files.
• Related suites green: test_provider_openai_responses +
  test_provider_selector (56 passed), test_provider_stream_contract +
  test_gateway_config_default_provider (43 passed).
• Full suite and webui build not run locally (no frontend changes; full
  suite includes live tests — leaving those to CI).

Linked Issues: None