fix(cursor_desktop): support delete on pre-table schemas and KV-only sessions

[Adityakk9031]

In CursorDesktopStore::delete(), the store ran an unconditional DELETE FROM composerHeaders WHERE composerId = ?1 and checked whether any rows were deleted from that table alone.

This caused two issues:

1. On older Cursor databases that lack the composerHeaders table entirely (which discover() explicitly supports by falling back to scanning cursorDiskKV), delete() failed with a SQLite error no such table: composerHeaders.
2. Even on modern databases with composerHeaders, if a session was discovered from cursorDiskKV without an accompanying header entry, delete() cleaned up the KV entries but returned Err(Error::Malformed("no such session")) because deleted from composerHeaders was 0.

This change:

• Checks has_table(&conn, "composerHeaders")? before attempting deletion from composerHeaders.
• Counts both deleted header rows and deleted KV keys.
• Only returns Error::Malformed when both counts are 0, allowing pre-table databases and KV-only sessions to be deleted cleanly.

Testing

• Added delete_roundtrip_removes_session_and_fails_on_duplicate.
• Added delete_succeeds_on_database_without_composer_headers_table.
• Added delete_succeeds_on_kv_only_session_in_modern_database.
• cargo test -p txcript --no-default-features --features opencode,hermes,search --test integration cursor_desktop (12/12 passed).
• cargo clippy -p txcript --no-default-features --features opencode,hermes,search --all-targets -- -D warnings (clean).