txnkv: support shared lock upgrades

[wfxr]

Summary

Support upgrading a pessimistic shared lock to an exclusive lock when the caller explicitly opts in through LockCtx.AllowSharedLockUpgrade.

This is needed for TiDB foreign-key checking with tidb_foreign_key_check_in_shared_lock enabled. In that flow, a transaction may first acquire a shared lock while checking the parent row and then update the same parent row later in the same transaction.

Related to pingcap/tidb#68815.

Changes

• Add LockCtx.AllowSharedLockUpgrade to gate shared-to-exclusive lock upgrades explicitly.
• Detect keys already locked in shared mode and send upgrade requests separately from normal exclusive-lock requests.
• Promote local lock flags only after a successful upgrade.
• Preserve the existing shared-lock state when TiKV returns deterministic upgrade conflicts or validation errors.
• Mark the transaction result as undetermined when an upgrade failure leaves the remote lock state uncertain, preventing later lock or commit attempts from continuing with divergent local/remote lock state.
• Add typed handling for LockUpgradeConflict errors and API v2 key decoding support.
• Reject shared-lock upgrade while aggressive/fair locking mode is active.

Dependency / TODO

This draft currently depends on kvproto support for kvrpcpb.LockUpgradeConflict:

• feat(kvrpcpb): add lock upgrade conflict error pingcap/kvproto#1495

Temporary TODO before marking this PR ready:

• Wait for feat(kvrpcpb): add lock upgrade conflict error pingcap/kvproto#1495 to merge.
• Update github.com/pingcap/kvproto in go.mod to the merged commit that contains LockUpgradeConflict.
• Remove the temporary go.work / go.work.sum workspace files after the dependency is updated.
• Rerun the related client-go tests without the local kvproto workspace.

Testing

Attempted:

go test ./error ./internal/apicodec ./txnkv/transaction

Blocked before running tests because the temporary go.work declares go 1.25.9 while go.mod requires go 1.25.10.

Also attempted:

GOWORK=off GOCACHE=/tmp/codex-go-cache go test ./error ./internal/apicodec ./txnkv/transaction

This confirmed the branch currently depends on the pending kvproto change and fails to compile without it:

undefined: kvrpcpb.LockUpgradeConflict
keyErr.LockUpgradeConflict undefined

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