[codex] Add web search grounding

[Kamilbenkirane]

Summary

Adds normalized web-search grounding to text outputs: searched queries, sources, and span citations.

This wires the shared Grounding model into output assembly and adds native extraction plus text-level mapping for providers/protocols that expose structured search data.

Details

• Adds shared grounding models and output hooks for sync and streaming text generation.
• Maps structured citations for OpenResponses, Chat Completions-compatible providers, Google Gemini, Anthropic Messages, and Groq Compound search results.
• Keeps native provider/protocol layers responsible for raw wire extraction and text modality layers responsible for normalized Grounding construction.

Validation

• uv run ruff check src/celeste
• uv run mypy src/celeste
• git diff --check
• focused smoke assertions for xAI numeric titles, Google byte-to-character offsets, Anthropic citation spans, and Groq search rows
• pre-commit/pre-push hooks passed, including tests with coverage