pageserver: various import flow fixups

[VladLazar]

Problem

There's a bunch of TODOs in the import code.

Summary of changes

1. Bound max import byte range to 128MiB. This might still be too high, given the default job concurrency, but it needs to be balanced with going back and forth to S3.
2. Prevent unsigned overflow when determining key range splits for concurrent jobs
3. Use sharded ranges to estimate task size when splitting jobs
4. Bubble up errors that we might hit due to invalid data in the bucket back to the storage controller.
5. Tweak the import bucket S3 client configuration.

[github-actions]

8525 tests run: 7942 passed, 0 failed, 583 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_sharding_split_failures[failure2]: debug-x86-64-without-lfc

Postgres 15

• test_sharding_split_failures[failure9]: release-arm64-with-lfc
• test_sharding_split_failures[failure5]: release-arm64-with-lfc

Postgres 14

• test_versions_mismatch[combination_nnnnn]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.3% (9068 of 28076 functions)
• lines: 48.6% (79963 of 164567 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c227269 at 2025-05-30T12:42:22.581Z :recycle:}