[codex] Add exact session lookup mode

[canvrno-oai]

This PR adds an exact-name search mode to thread/list and uses it for startup resume/fork name lookups. The change is primarily a code cleanup: it keeps the exact-match decision in the app-server path instead of finishing that filtering in the TUI client.

What changed:

• Adds a searchMode option to ThreadListParams with contains as the default behavior and exact for name lookups.
• Applies exact filtering in the app-server while preserving the existing thread ordering and source/archive filters.
• Updates the TUI startup resume/fork lookup to request exact matches from thread/list.
• Regenerates the app-server schema and TypeScript protocol fixtures.
• Updates existing app-server/TUI tests and call sites for the new request field.

Testing:

• Ran just fmt from codex-rs.
• Ran just write-app-server-schema from codex-rs.
• Ran git diff --check 87d2235b5470d8969b51d80278a46aa30703eeab.

[canvrno-oai]

@codex review

[chatgpt-codex-connector]

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