GH-51114: [FlightSQL][C++] Expose is_update field of PreparedStatement

[SHIVANSH-ux-ys]

Fixes #51114.

Exposes the is_update boolean field in the C++ Flight SQL layer:

• Added is_update field to ActionCreatePreparedStatementResult in server.h and updated PackActionResult in server.cc.
• Added bool is_update = false parameter and is_update() const accessor to PreparedStatement in client.h/client.cc.
• Updated PreparedStatement::ParseResponse to extract is_update.
• Updated SQLite example server (sqlite_server.cc) to populate is_update via sqlite3_stmt_readonly.
• Added assertions in server_test.cc for both query (ASSERT_FALSE) and update (ASSERT_TRUE) prepared statements.

• GitHub Issue: [FlightSQL][C++] Expose is_update field of PreparedStatement #51114

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⚠️ GitHub issue #51114 has been automatically assigned in GitHub to PR creator.

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[lidavidm]

Seems reasonable, but please fix the PR title/description.

Also CC @ennuite. Was there an integration test case we can enable? (I don't see one.)

[SHIVANSH-ux-ys]

Thanks @lidavidm! Updated the PR title and description.

Also added support to populate is_update in the SQLite example server (sqlite_server.cc) using sqlite3_stmt_readonly and added integration test assertions in server_test.cc for both TestCommandPreparedStatementQuery (ASSERT_FALSE) and TestCommandPreparedStatementUpdate (ASSERT_TRUE). CC @ennuite

[github-actions]

⚠️ GitHub issue #51114 has been automatically assigned in GitHub to PR creator.

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[ennuite]

I will do a deeper review tomorrow as I'm still familiarizing myself with the C++ codebase.

However, I don't see the same semantics as other PRs here. We need more than is_update being true/false, we need a presence check to know if the field was ever sent. This is important for backward compatibility (I assume most servers will not send this field).

The updates done to the tests also seem to not leave any test that doesn't set the field. IMO this is the most important test (again for backward compatibility).

@SHIVANSH-ux-ys I would like to preserve the true/false/unset semantics if possible.

[ennuite]

Thanks for the cc @lidavidm

[SHIVANSH-ux-ys]

Updated with the Doxygen parameter docstring for is_update in client.h. Ready for review / workflow approval when convenient @lidavidm @ennuite!