fix(udb): add txn tagging

[MasterPtato]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

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Checklist:

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• My changes generate no new warnings
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[MasterPtato]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore: bump rivetkit protocol version #4329
• fix(udb): add txn tagging #4328 👈 (View in Graphite)
• fix(api): make actor destroy wait for ack #4327
• chore: flatten runtime config #4326
• chore: deploy patch #4324
• main

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[claude]

PR Review: fix(udb): add txn tagging

This is a small, focused change that adds transaction tagging support to the UniversalDB abstraction layer.

Summary

The PR adds a tag method to the TransactionDriver trait with a default no-op implementation, and exposes it through the public Transaction::tag API.

Whats Good

• Correct approach for opt-in functionality. Providing a default no-op implementation on the trait is the right pattern for an optional capability, existing driver implementations (RocksDB, PostgreSQL) do not need to change, and future drivers (e.g. a FoundationDB driver) can override it.
• Clean and minimal. The change is exactly as large as it needs to be, with no unnecessary complexity.
• Consistent delegation. Transaction::tag correctly delegates to self.driver.tag(tag), consistent with how other methods in this file are structured.

Observations / Minor Notes

1. The comment on the trait default is slightly awkward.
   The comment 'No-op unless implemented' is a bit off, the method is implemented (as a no-op). A clearer phrasing would be 'Default no-op. Override to attach a tag to the transaction for profiling or throttling.'

2. Single-string tag vs. key-value pairs.
   Most tag systems (including FoundationDB transaction tags) use a single opaque byte string, so tag takes a str which is the right signature for FDB. The existing gasoline builder uses .tag(k, v) key-value pairs, but those are independent systems with no conflict. Worth noting if callers ever want key-value semantics here in the future.

3. No call sites in this PR.
   The method is introduced but not called anywhere in this diff. This appears to be preparatory infrastructure for a downstream PR in the Graphite stack, which is a reasonable approach.

4. InformalTransaction does not expose tag.
   Transaction now has tag, but InformalTransaction does not. If the downstream PR needs to tag through an informal transaction, this would need to be added. Not a blocker.

Verdict

LGTM. The change is correct, minimal, and follows existing patterns. The default no-op on the trait is the right extensibility mechanism for this kind of optional driver capability.

[pkg-pr-new]

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commit: 5ea9e02

[MasterPtato]

This change is part of the following stack:

• chore: deploy patch #4324
  • chore: flatten runtime config #4326
    • fix(api): make actor destroy wait for ack #4327
      • fix(udb): add txn tagging #4328 ◀
        • chore: bump rivetkit protocol version #4329
          • fix(ups): formalize subjects #4334
            • fix(api): properly dedup name list responses #4337
              • fix(pb): clear actor reschedule ts on alloc #4338

_{Change managed by git-spice.}