perf(txn): avoid default event callback allocations

[VioletQwQ-0]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #25472

What this PR does / why we need it:

Every new transaction currently builds a callback map and slice to install the
two client-owned ClosedEvent callbacks. Under the 1000-VU point-select
workload, AppendEventCallback accounted for about 2.65 GB of allocation in a
30-second profile.

This change stores the immutable default close callbacks once on txnClient.
Transaction operators share them until a caller adds a custom callback, at
which point the operator allocates its own callback map using copy-on-write.
Default callbacks still run before custom callbacks and preserve first-error,
commit, rollback, and restart behavior.

Changes:

• share immutable client-owned default ClosedEvent callbacks;
• allocate an operator callback map only when custom callbacks are appended;
• reset custom callbacks when an operator is reused;
• add ordering, isolation, close-once, restart, race, and allocation coverage.

Benchmark results on Apple M1 (-benchmem, five runs; allocation values were
stable across all runs):

Benchmark	Before	After
New + rollback	2544 B / 17 allocs	2000 B / 12 allocs
New + one callback + rollback	2664 B / 19 allocs	2528 B / 17 allocs
New + 14 trace callbacks + rollback	5976 B / 50 allocs	4088 B / 46 allocs

Clean remote point-select A/B/A at 1000 VU:

• baseline A: 180,508.66 TPS;
• patched: 188,905.01 TPS;
• baseline B: 178,377.58 TPS;
• patched profile run: 186,083.03 TPS versus 180,208.74 TPS control;
• AppendEventCallback disappeared from the patched allocation profile without
  an equivalent allocation transfer.

Validation:

• go test -count=1 ./pkg/txn/client
• go test -race -count=1 ./pkg/txn/client -run '^(TestEventCallbacksDefaultOrderAndFirstError|TestDefaultEventCallbacksCopyOnWrite|TestClientClosedEventDefaultsRunBeforeCustomOnce|TestRestartTxnRestoresDefaultsAndDropsCustomCallbacks|TestClosedEvent|TestCommitClosedEventWhenWaitRunningSQLFails|TestRollbackClosedEventWhenWaitRunningSQLFails)$'
• go test -count=1 ./pkg/frontend ./pkg/sql/compile
• go test -run '^$' -bench '^(BenchmarkTxnNewRollback|BenchmarkTxnNewAppendCallbackRollback|BenchmarkTxnNewAppendTraceCallbacksRollback)$' -benchmem -count=5 ./pkg/txn/client

[XuPeng-SH]

Reviewed from lifecycle, concurrency, error-path, compatibility, and performance angles.

The client-owned defaults are initialized before publication and remain immutable; operators share only the read-only container, while the first custom append performs copy-on-write under the operator lock. Default-before-custom ordering, first-error behavior, close-once semantics, RestartTxn cleanup, failed creation, and snapshot/mirror behavior are preserved. The reset path also avoids retaining custom callback backing arrays across operator reuse.

Local validation on 54f0738:

• pkg/txn/client: count=3 PASS
• pkg/txn/client under -race: PASS, including the 50x100 concurrent New/Rollback lifecycle test
• focused callback lifecycle tests under -race count=3: PASS
• pkg/frontend and pkg/sql/compile compile tests: PASS
• allocation benchmarks reproduce 2000 B/12 allocs, 2528 B/17 allocs, and 4088 B/46 allocs for the three documented paths

No blocking correctness or robustness issue found. CI is still running and should remain a merge prerequisite.

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