fix: set SQLite WAL pragmas before transactions in etl-indexer

[jverdicc]

Motivation

• Tests were failing with SQLite errors because PRAGMA journal_mode=WAL was being executed inside an active transaction, which SQLite disallows.
• The intent is to ensure WAL and related safety pragmas are applied on the raw connection before any transaction begins so schema DDL and migrations run without hitting journal/safety mode constraints.

Description

• Removed PRAGMA calls from apply_schema so the schema DDL can run inside transactions without attempting to change journal/safety mode.
• Added a new helper apply_connection_pragmas(conn: &Connection) that runs PRAGMA journal_mode=WAL;, PRAGMA synchronous=FULL;, and PRAGMA foreign_keys=ON; on the opened connection.
• Call apply_connection_pragmas immediately after Connection::open(...) in both migrate_schema and build_index, before conn.transaction() is created.
• Drop the opened Connection (drop(conn)) after committing the transaction in build_index so WAL-backed changes are flushed before renaming the temporary DB file.

Testing

• Ran the crate tests with cargo test -p evidenceos-etl-indexer and the previously failing test tests::creates_expected_indexes_and_query_results now passes (all crate tests passed).
• Ran formatting and lints with cargo fmt --all && cargo fmt --check and cargo clippy --workspace --all-targets -- -D warnings, both succeeded with no errors.
• Ran full workspace tests with cargo test --workspace and observed no failing tests (FINAL:0).

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Codex Task