[codex] Make thread search case-insensitive

[fc-oai]

Summary

• make rollout content search prefilter rollout files case-insensitively
• keep the no-ripgrep fallback scan and visible snippet matcher aligned with that behavior
• cover a lowercase thread/search query matching mixed-case conversation content

Why

The rollout-backed thread/search path used exact string matching in both its rg prefilter and semantic snippet generation. A content result could be missed solely because the query casing did not match the stored conversation text.

Validation

• just fmt
• cargo test -p codex-app-server thread_search_returns_content_matches
• cargo test -p codex-rollout
• just bazel-lock-update
• just bazel-lock-check
• cargo build -p codex-cli
• launched a local Electron dev instance with the rebuilt CLI binary