implement POST /v1/transactions/commit endpoint

[laskoviymishka]

Feature Request / Improvement

Implement CommitTransaction() on the REST catalog, satisfying the TransactionalCatalog interface.

Wire Format

POST /v1/{prefix}/transactions/commit
Content-Type: application/json

{
  "table-changes": [
    {
      "identifier": { "namespace": ["db"], "name": "t1" },
      "requirements": [
        { "type": "assert-table-uuid", "uuid": "..." },
        { "type": "assert-ref-snapshot-id", "ref": "main", "snapshot-id": 123 }
      ],
      "updates": [
        { "action": "add-snapshot", ... },
        { "action": "set-snapshot-ref", ... }
      ]
    },
    {
      "identifier": { "namespace": ["db"], "name": "t2" },
      "requirements": [ ... ],
      "updates": [ ... ]
    }
  ]
}

Response: 204 No Content

Error Handling

Status	Error	Meaning
404	ErrNoSuchTable	A referenced table doesn't exist
409	ErrCommitFailed	One or more requirements failed — entire transaction rejected
500, 502, 504	ErrCommitStateUnknown	Server error — commit may or may not have been applied
503	ErrCommitStateUnknown	Service unavailable

On 409, no tables are modified — all-or-nothing semantics.

Implementation Notes

• Reuse existing doPostAllowNoContent helper with allowNoContent=true (expects 204)
• Same error mapping pattern as CommitTable (rest.go:828-835)
• JSON field name on the wire is "table-changes" (kebab-case, matching Java's CommitTransactionRequestParser)
• Each entry in table-changes reuses the existing identifier, requirements, and updates serialization
• No response body to parse
• No retry logic built in (matches Java behavior — retries are the caller's responsibility)

Validation

• commits list must not be empty
• Every commit must have a non-nil Identifier (unlike single-table commit where the identifier is in the URL path)

References

• Iceberg REST OpenAPI spec
• Java CommitTransactionRequest
• Java CommitTransactionRequestParser

Depends On

• TransactionalCatalog interface and TableCommit type add TransactionalCatalog interface for multi-table atomic commits #784