[Docs] Add unbounded source guidance to the Python I/O connector guide

[Eliaaazzz]

Adds a "Reading from an unbounded source" section to the Python I/O connector development guide.

The guide currently documents only the bounded BoundedSource / RangeTracker path and does not say how to build an unbounded (streaming) source in Python. The Java guide covers this with its UnboundedSource / UnboundedReader sections. The Python SDK has no UnboundedSource class; an unbounded source is written as a Splittable DoFn, but the I/O connector guide, which is where the Java unbounded documentation lives, had no Python counterpart.

What changes

• A note under "Implementing the Source interface" stating that the Source interface is bounded only, linking to the new section.
• A new "Reading from an unbounded source" section with a minimal unbounded Splittable DoFn example (@beam.DoFn.unbounded_per_element(), RestrictionParam, WatermarkEstimatorParam, try_claim, defer_remainder).
• A mapping from each UnboundedSource / UnboundedReader concern in the Java guide to its Splittable DoFn equivalent: initial and dynamic splitting, checkpointing through residual restrictions, resuming through defer_remainder, watermark estimators, deduplication, and drain through RestrictionProvider.truncate.
• Pointers to the reusable building blocks: PeriodicImpulse / PeriodicSequence and the experimental Watch transform added in [Python] Add Watch transform with growth_of polling SDF #39023.

The example follows the same self-checkpoint pattern as the SDF_UserInitiatedCheckpoint snippet in the programming guide and periodicsequence.py: call restriction_tracker.defer_remainder(...) and return, which is the mechanism the portable runners implement.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

Assigning reviewers:

R: @damccorm for label website.

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).