feat(mysql): add aggregate pushdown support for MySQL FDW

[burmecia]

What kind of change does this PR introduce?

This PR is to add aggregate pushdown support for MySQL FDW.

What is the current behavior?

Aggregate pushdown support was added to framework in #586, but MySQL FDW doesn't support it yet.

What is the new behavior?

Added aggregate pushdown support for MySQL FDW, which supports below forms of aggregates.

Supported aggregates

count(*), count(col), count(distinct col), sum(col), avg(col), min(col), max(col).

Supported shapes

scalar aggregates, group by over plain columns, with or without a where clause. Pushdown also works when the foreign table option is a sub-query.

Additional context

N/A

[Copilot]

Pull request overview

This PR adds aggregate pushdown support to the MySQL FDW so common aggregate queries can be executed remotely in MySQL instead of being aggregated locally in Postgres. It extends the new aggregate-pushdown framework introduced in #586 and updates the MySQL FDW implementation, tests, and docs accordingly.

Changes:

• Add MySQL-specific aggregate SQL deparsing and implement the aggregate FDW hooks (supported_aggregates, supports_group_by, begin_aggregate_scan).
• Add end-to-end tests for supported aggregate pushdown behavior and a couple of non-pushdown cases.
• Update MySQL FDW versioning and documentation to advertise aggregate pushdown support.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
wrappers/src/fdw/mysql_fdw/tests.rs	Adds aggregate pushdown integration tests for MySQL FDW.
wrappers/src/fdw/mysql_fdw/mysql_fdw.rs	Implements aggregate pushdown query generation and execution for MySQL.
wrappers/src/fdw/mysql_fdw/README.md	Bumps changelog/version notes for the MySQL FDW.
docs/catalog/mysql.md	Documents aggregate pushdown support, limits, and examples.
CLAUDE.md	Updates contributor/developer guidance to mention aggregate pushdown hooks and support matrix.

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