Return standalone web search previews to clients

[alexi-openai]

Summary

• parse URL, title, and snippet previews from standalone /alpha/search output
• deduplicate and bound previews before attaching them to the client-facing web search item
• persist and restore those previews through the foundation API

Stacked on #31515.

Model-context boundary

The standalone search function output sent back to the model is unchanged. Parsed previews are attached only to WebSearchItem.results for app rendering and persistence; this PR does not change ResponseItem, WebSearchAction, or Responses request construction.

Validation

• just test -p codex-web-search-extension search_results (red before implementation, green after)
• just test -p codex-web-search-extension
• just test -p codex-app-server standalone_web_search_round_trips_output
• just fix -p codex-web-search-extension -p codex-app-server
• just fmt

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate result headers before splitting snippets

When a standalone search snippet contains a line ending in ordinary parentheses, such as ... (Reuters), this delimiter treats it as the next result header before any URL validation. The parser then rejects that bogus header later and continues scanning for the next header, which drops that snippet line and any following non-header lines from the rendered preview. Search snippets are arbitrary source text, so only validated http(s) result headers should terminate snippet collection.

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