[Python] Only give an unknown option a value that directly follows it

[Thanatos9404]

Fixes #22434

PipelineOptions.get_all_options(retain_unknown_options=True) decided whether an unknown flag takes a value by looking at the next entry in the list of unknown args. argparse takes the known options out of that list, so entries that are next to each other there aren't always next to each other in the original flags. For --unknown_flag --job_name=foo bar, --unknown_flag was registered as taking a value and parsing failed with expected one argument, which also broke Pipeline.run().

Now an unknown flag only takes a value when the value directly follows it in the original flags. Cases that already worked behave as before. One related case changes: --flag=value followed by a stray argument now keeps its inline value (the input that broke the load tests in #22422).

Tests: new cases in pipeline_options_test.py, six of which fail without the change and pass with it. I also added a CHANGES.md entry under Bugfixes for 2.78.0.

[github-actions]

📋 Review Summary

This PR fixes a bug in PipelineOptions.get_all_options(retain_unknown_options=True) where non-adjacent unknown arguments were erroneously paired together after known arguments were stripped by argparse. By tracking the original indices of unknown arguments in the raw flag list, it ensures an unknown flag only consumes a value that directly follows it and properly handles --flag=value options followed by stray positional arguments.

🔍 General Feedback

• The implementation in _find_positions and get_all_options is clean, concise, and handles edge cases (like -- separators, single-dash options, inline = values, and known positionals) accurately.
• The test coverage added in pipeline_options_test.py is thorough and directly verifies all relevant flag combinations.
• One minor edge case to be aware of (not introduced by this PR, as self._flags is already re-parsed multiple times): if a caller passes a single-use iterator (e.g., a generator) as flags to PipelineOptions(flags), it would be exhausted during __init__. Normalizing self._flags to a list in __init__ could be a nice future hardening, though out of scope for this fix.

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[github-actions]

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