store-sqlite: treat substring queries as literals

[lsj5031]

context/problem

search_substring() builds a SQL LIKE pattern directly from user text. Queries containing _ or % are interpreted as wildcards, so literal searches (for example hello_world) can match unrelated messages (like helloxworld). This makes fallback search results noisy and unpredictable.

exact change

• Added escape_like_pattern() in crates/store-sqlite/src/lib.rs to escape %, _, and \ in user queries.
• Updated search_substring() to use the escaped pattern and ESCAPE '\\' in SQL.
• Added regression test substring_search_escapes_like_wildcards to verify hello_world matches literal underscore content only.

validation steps

• cargo fmt --all --check
• cargo test -p store-sqlite
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

All commands passed locally.

rollback notes

Revert commit 3122ce2 to restore previous behavior where % and _ act as SQL wildcards in substring fallback search.