test + refactor: staging

[de-sh]

Fixes #XXXX.

Description

While trying to audit the code to perform staging, I made these improvements that reduces unnecessary copies and added tests to critical parts of the flow and hence improve coverage.

---

This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

[coveralls]

Pull Request Test Coverage Report for Build 13032630803

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 426 of 618 (68.93%) changed or added relevant lines in 16 files are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+2.4%) to 15.332%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/handlers/http/modal/ingest/ingestor_logstream.rs	0	1	0.0%
src/migration/metadata_migration.rs	0	1	0.0%
src/sync.rs	0	1	0.0%
src/handlers/http/health_check.rs	0	2	0.0%
src/handlers/http/logstream.rs	0	2	0.0%
src/handlers/http/modal/query/querier_logstream.rs	0	2	0.0%
src/metadata.rs	0	2	0.0%
src/query/stream_schema_provider.rs	0	2	0.0%
src/handlers/airplane.rs	0	4	0.0%
src/utils/mod.rs	33	37	89.19%

Files with Coverage Reduction	New Missed Lines	%
src/handlers/http/health_check.rs	1	0.0%
src/query/stream_schema_provider.rs	1	26.71%
src/handlers/airplane.rs	3	0.0%

Totals
Change from base Build 13006822034:	2.4%
Covered Lines:	2939
Relevant Lines:	19169

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💛 - Coveralls

[de-sh]

Only failure is because of alerts check, which will continue to fail until #1101 is merged

[parmesant]

LGTM