proxy: do not invalidate cache for permit errors

[conradludgate]

Problem

If a permit cannot be acquired to connect to compute, the cache is invalidated. This had the observed affect of sending more traffic to ProxyWakeCompute on cplane.

Summary of changes

Make sure that permit acquire failures are marked as "should not invalidate cache".

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

[stradig]

LGTM

[github-actions]

3024 tests run: 2891 passed, 0 failed, 133 skipped (full report)

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Code coverage* (full report)

• functions: 31.2% (6257 of 20059 functions)
• lines: 46.7% (46939 of 100613 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
74d7a9a at 2024-05-08T10:41:30.990Z :recycle:}