feat(kvrpcpb): add lock upgrade conflict error

[wfxr]

What problem does this PR solve?

Cloud Storage Engine needs to return typed errors when a shared-lock upgrade request conflicts with an existing in-flight upgrader. Today the protocol has no dedicated KeyError variant for this case, so CSE cannot distinguish duplicate in-flight upgrade attempts from second-upgrader conflicts at the API boundary.

Related CSE work:

• tidbcloud/cloud-storage-engine#5476
• tidbcloud/cloud-storage-engine#5477

Related TiDB work:

• Support shared-lock upgrade in foreign key check tidb#68815

What is changed and how it works?

• Add LockUpgradeConflict to kvrpcpb.KeyError.
• Add LockUpgradeConflict.Reason with:
  • DuplicateInFlight
  • SecondUpgrader
• Regenerate Go protobuf output for kvrpcpb.
• Update scripts/proto.lock.

This is an additive protobuf change and preserves wire compatibility for existing clients.

Check List

• Updated protobuf schema.
• Regenerated checked-in Go protobuf output.
• Updated scripts/proto.lock.
• Verified Go generation and build with make go.
• Verified Rust generation paths with make rust.

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