feat!(c/driver/postgresql): defer transaction start

[lidavidm]

• Use ROLLBACK and not ROLLBACK AND CHAIN, which unexpectedly preserves transaction properties
• Lazily start transactions instead of eagerly
  • For pooled connections, this is better behavior
  • We do make slightly more server round-trips

Closes #4321.

Assisted-by: GPT-5.5 codex@openai.com

[Copilot]

Pull request overview

This PR updates the PostgreSQL C driver’s transaction behavior to avoid implicitly chaining transactions on COMMIT/ROLLBACK and to start transactions lazily (on first use) when autocommit is disabled—improving behavior with pooled connections.

Changes:

• Switch COMMIT; BEGIN TRANSACTION → COMMIT and ROLLBACK AND CHAIN → ROLLBACK.
• Add PostgresConnection::EnsureTransaction() and invoke it before statement execution and selected metadata calls when autocommit is disabled.
• Update/add Python and C++ tests to reflect “idle after commit/rollback” and to validate non-chaining rollback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
python/adbc_driver_postgresql/tests/test_dbapi.py	Updates txn-status expectations; adds a cross-connection transaction behavior test.
c/driver/postgresql/statement.cc	Ensures a transaction exists before executing queries/ingest when autocommit is disabled.
c/driver/postgresql/postgresql_test.cc	Adjusts txn-status assertions; adds tests for non-chaining rollback and lazy txn start in GetObjects.
c/driver/postgresql/connection.h	Exposes EnsureTransaction() to PostgresStatement via friendship.
c/driver/postgresql/connection.cc	Implements lazy transaction start; changes COMMIT/ROLLBACK semantics; updates autocommit toggle behavior and adds EnsureTransaction calls in metadata paths.

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