feat(bigquery): add typed handling for AI scalar functions

[RedZapdos123]

Description:

• add typed expression nodes for BigQuery AI scalar calls:
  • AI.EMBED -> AIEmbed
  • AI.SIMILARITY -> AISimilarity
  • AI.GENERATE -> AIGenerate
• keep SQL output backward compatible through explicit SQL names (EMBED, SIMILARITY, GENERATE)
• preserve generic behavior for unqualified scalar names; typed conversion is applied to dotted AI.* calls in BigQuery parser column-op handling
• add BigQuery dialect tests asserting both identity round-trip and typed AST nodes

Closes #7478.

Validation:

• python -m pytest -q tests/dialects/test_bigquery.py -k ml_functions -vv
• python -m pytest -q tests/dialects/test_bigquery.py -vv
• python -m pytest -q

Results:

• focused BigQuery ml_functions: passed.
• full BigQuery dialect module: passed (58 passed).
• full suite: passed (1076 passed).

The validation screenshots of the tests run, locally on WSL:

Additionally, verified direct parses now produce typed function nodes under AI.* dot calls while preserving round-trip SQL.