Brief summary of the change made

Fixes #8189.

BigQuery pipe-syntax AGGREGATE and EXTEND clauses take a
comma-delimited expression list, like SELECT and GROUP BY, but their
grammar wrapped that list without Indent / Dedent metadata. As a
result LT02 (layout.indent) flagged every continuation line as "Line
should not be indented", and sqlfluff fix would not indent a
multi-line clause — so an indented multi-line AGGREGATE was impossible.

This wraps the Delimited(...) list of both AggregateClauseSegment
and ExtendClauseSegment in Indent / Dedent, matching how
GroupAndOrderByClauseSegment (the GROUP BY used in the same pipe
syntax) already behaves.

Before:

FROM my_table
|> AGGREGATE
    SUM(a) AS total_a,   -- LT02: Line should not be indented
    SUM(b) AS total_b    -- LT02: Line should not be indented
GROUP BY c

After: lints clean, and fix indents a flat list to the above.

Are there any other side effects of this change that we should be aware of?

Indent / Dedent are meta segments and are excluded from the parse
YAML fixtures, so existing test/fixtures/dialects/bigquery fixtures are
unchanged. All 429 bigquery dialect tests and the LT01/LT02/LT03 rule
suites pass. Single-line AGGREGATE / EXTEND are unaffected.

Pull Request checklist

• Included test cases to demonstrate the code change:
  • .yml rule test cases in test/fixtures/rules/std_rule_cases
    (LT02-indent.yml): a passing indented multi-line AGGREGATE, a
    fail/fix pair, and a passing multi-line EXTEND.
• Change is small and self-documenting; no docs change needed.
• Created GitHub issues for any relevant followup — n/a.

Summary by cubic

Fix indentation for BigQuery pipe AGGREGATE and EXTEND clause lists. Adds indent metadata so multi-line lists are indented correctly, removing LT02 false positives and allowing sqlfluff fix to format them.

• Bug Fixes
  • Wrap the Delimited(...) expression list in Indent/Dedent for both clauses, matching pipe GROUP BY.
  • Add LT02 rule cases for multi-line AGGREGATE and EXTEND, including a fixable failure.

^{Written for commit ef0bfab. Summary will update on new commits.}