fix(bigquery): parse FOR SYSTEM TIME AS OF (with spaces) [CLAUDE]

[lawrence3699]

Closes #7481

BigQuery accepts both FOR SYSTEM_TIME AS OF and FOR SYSTEM TIME AS OF (with spaces instead of underscore). The parser only recognized the underscore form, raising ParseError: Invalid expression / Unexpected token for the space form.

Before: SELECT * FROM tbl FOR SYSTEM TIME AS OF z → ParseError
After: SELECT * FROM tbl FOR SYSTEM TIME AS OF z → parses correctly, normalizes to FOR SYSTEM_TIME AS OF in output

The fix adds "FOR SYSTEM TIME" as an additional keyword mapping to TIMESTAMP_SNAPSHOT in the BigQuery tokenizer, alongside the existing "FOR SYSTEM_TIME" entry.

Validation: BigQuery dialect tests pass (python -m pytest tests/dialects/test_bigquery.py).

[Copilot]

Pull request overview

Extends the BigQuery dialect tokenizer to recognize BigQuery’s alternative time-travel syntax FOR SYSTEM TIME AS OF (spaces instead of underscore), so it parses successfully and round-trips normalized output as FOR SYSTEM_TIME AS OF.

Changes:

• Added FOR SYSTEM TIME as an additional BigQuery keyword mapped to TokenType.TIMESTAMP_SNAPSHOT.
• Added a BigQuery dialect identity test asserting parsing and normalization from FOR SYSTEM TIME → FOR SYSTEM_TIME.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sqlglot/dialects/bigquery.py	Adds a multi-word keyword mapping so the tokenizer emits TIMESTAMP_SNAPSHOT for FOR SYSTEM TIME.
tests/dialects/test_bigquery.py	Adds coverage ensuring the spaced form parses and normalizes to the underscore form.

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