Delay BigQueryToMsSqlOperator source table parsing

[fat-catTW]

Move BigQueryToMsSqlOperator parsing of source_project_dataset_table out of __init__ so templated values are rendered before validation and dataset/table extraction.

This also removes the corresponding validate_operators_init exemption and adds regression coverage for rendered source table values.

related: #70296

Was generative AI tooling used to co-author this PR?

• [X] Yes (please specify the tool below)

Generated-by: [Codex] following the guidelines

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[potiuk]

Thanks — and the test here is the strongest in this whole batch. test_execute_uses_rendered_source_project_dataset_table calls operator.render_template_fields({...}) and then execute, asserting on the resulting list_rows and BigQueryTableLink.persist arguments. That exercises the real rendering path rather than simulating it by assigning attributes, so it would catch a field being dropped from template_fields — several of the sibling PRs in this batch fake the rendering step instead. I've pointed #70530 at this as the pattern to copy.

Extracting _get_source_project_dataset_table_parts() rather than duplicating the split in two places is right, and keeping the ValueError with from None preserves the clean error message.

One question inline about the placeholder.

---

Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting

[github-actions]

Backport failed to create: v3-3-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-3-test

You can attempt to backport this manually by running:

cherry_picker e6341f3 v3-3-test

This should apply the commit to the v3-3-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.