feat: update providers and add A2A session-scoped LLM headers

[guima-why]

Summary

• update provider model catalogs, defaults, fallbacks, multimodal metadata, and reasoning-effort options across CLI and Web
• add session-scoped A2A llm_headers forwarding for provider requests
• protect A2A header metadata from task history and telemetry exposure, including permission pause/resume flows
• refresh the provider update guide and regression coverage

Validation

• make test (16032 passed, 2 skipped)
• make lint
• translation extraction/update/compile checks