Refactor: Change ALTER FRAGMENT SET RATE_LIMIT to support specific rate limit types

[Copilot]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

The previous ALTER FRAGMENT SET RATE_LIMIT had ambiguous semantics when a fragment has both sink and backfill rate limits. This PR decouples the single RATE_LIMIT into four specific types.

New syntax:

ALTER FRAGMENT <id> SET BACKFILL_RATE_LIMIT TO <value>;
ALTER FRAGMENT <id> SET SOURCE_RATE_LIMIT TO <value>;
ALTER FRAGMENT <id> SET DML_RATE_LIMIT TO <value>;
ALTER FRAGMENT <id> SET SINK_RATE_LIMIT TO <value>;

Changes:

• AlterFragmentOperation enum: replaced AlterBackfillRateLimit with four specific variants (SetBackfillRateLimit, SetSourceRateLimit, SetDmlRateLimit, SetSinkRateLimit)
• Parser: reuses existing rate limit parsing methods instead of a separate parse_alter_fragment_rate_limit
• Handler: all four types dispatch to the same PbThrottleTarget::Fragment backend

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note

Breaking change: ALTER FRAGMENT <id> SET RATE_LIMIT syntax has been replaced with specific rate limit types:

• ALTER FRAGMENT <id> SET BACKFILL_RATE_LIMIT TO <value>
• ALTER FRAGMENT <id> SET SOURCE_RATE_LIMIT TO <value>
• ALTER FRAGMENT <id> SET DML_RATE_LIMIT TO <value>
• ALTER FRAGMENT <id> SET SINK_RATE_LIMIT TO <value>

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Refactor: Change ALTER FRAGMENT SET RATE_LIMIT to ALTER FRAGMENT SET (BACKFILL_RATE_LIMIT | SOURCE_RATE_LIMIT | DML_RATE_LIMIT | SINK_RATE_LIMIT)</issue_title>
<issue_description>As per title. Currently we just lump it into a single alter fragment set rate limit, however if a fragment has both sink rate limit and backfill rate limit, the semantics of alter becomes ambiguous. It's better to decouple these.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Refactor: Change ALTER FRAGMENT SET RATE_LIMIT to ALTER FRAGMENT SET (BACKFILL_RATE_LIMIT | SOURCE_RATE_LIMIT | DML_RATE_LIMIT | SINK_RATE_LIMIT) #23996

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.