reinstate write-path key check

[jcsp]

Problem

In #8621, validation of keys during ingest was removed because the places where we actually store keys are now past the point where we have already converted them to CompactKey (i128) representation.

Summary of changes

Reinstate validation at an earlier stage in ingest. This doesn't cover literally every place we write a key, but it covers most cases where we're trusting postgres to give us a valid key (i.e. one that doesn't try and use a custom spacenode).

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[github-actions]

3857 tests run: 3743 passed, 0 failed, 114 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_scrubber_physical_gc[4]: release-x86-64

Code coverage* (full report)

• functions: 31.7% (7346 of 23155 functions)
• lines: 49.9% (59499 of 119179 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
05c7804 at 2024-09-09T17:03:08.601Z :recycle:}