safekeeper: fix OpenAPI spec

[bayandin]

Problem

Safekeeper's OpenAPI spec isn't correct:

Semantic error at paths./v1/tenant/{tenant_id}/timeline/{timeline_id}.get.responses.404.content.application/json.schema.$ref
$refs must reference a valid location in the document
Jump to line 126

Checked on https://editor.swagger.io/

Summary of changes

• Add NotFoundError
• Add description and license fields to make Cloud OpenAPI spec linter happy

Tested in https://github.com/neondatabase/cloud/pull/17654

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

[github-actions]

5020 tests run: 4856 passed, 0 failed, 164 skipped (full report)

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Flaky tests (7)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_hot_standby_feedback: debug-x86-64
• test_pg_regress[4]: debug-x86-64
• test_restart_endpoint_after_switch_wal: release-x86-64

Postgres 16

• test_subscriber_restart: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Postgres 14

• test_isolation[4]: release-arm64

Code coverage* (full report)

• functions: 31.8% (7420 of 23300 functions)
• lines: 49.8% (59730 of 119904 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6ced1dd at 2024-09-19T21:38:50.609Z :recycle:}