Refactor SLRU download interface between Postgres and the neon extension

[hlinnaka]

Move the responsibility of writing the SLRU segment contents to the neon extension. The neon extension hook now downloads the file and places it to the correct path, and returns a true/false to the caller to indicate if the file existed. On successful download, the caller retries opening the file.

Corresponding Postgres PRs:

• v14
• v15
• v16
• v17

[github-actions]

If this PR added a GUC in the Postgres fork or neon extension,
please regenerate the Postgres settings in the cloud repo:

make NEON_WORKDIR=path/to/neon/checkout \
  -C goapp/internal/shareddomain/postgres generate

If you're an external contributor, a Neon employee will assist in
making sure this step is done.

[hlinnaka]

This is in preparation for the new communicator implementation. That implementation will write the downloaded segment directly to the file, rather than a palloc'd buffer. This refactors the API to be like that even with the current communicator implementation, to keep minimize the differences between the two.

(The current implementation was slightly broken with the new communicator: SimpleLruDownloadSegment did pfree(NULL), which segfaults. That was the immediate impetus for doing this right now, but I've wanted to do this refactoring for the sake of tidiness anyway)

[github-actions]

9119 tests run: 8465 passed, 0 failed, 654 skipped (full report)

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Code coverage* (full report)

• functions: 34.7% (8840 of 25482 functions)
• lines: 45.7% (71640 of 156719 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1d59754 at 2025-07-31T16:35:36.670Z :recycle:}