Clarify MultiHostUrl behavior for Postgres DSN without host

[gcomneno]

Summary

This PR updates tests to document the current MultiHostUrl behavior for the Postgres DSN pattern:

postgresql://derp@/databasename?host=/var/run/postgresql

Today this is rejected with url_parsing: empty host.

What’s covered by the added cases

Accepted:

• postgresql:///databasename
• postgresql:///databasename?user=derp
• postgresql://derp@%2Fvar%2Frun%2Fpostgresql/databasename

Rejected:

• postgresql://derp@/databasename?host=/var/run/postgresql → empty host

Question (maintainer guidance)

Would you prefer MultiHostUrl to remain URL-strict here (i.e. keep rejecting userinfo + empty host), and keep any Postgres-DSN specific flexibility in a dedicated type/variant instead?

CI note

Some checks are currently failing, but this appears unrelated to this test-only change (integration error: SchemaValidator.__new__() got an unexpected keyword argument '_use_prebuilt', suggesting an environment/build mismatch). Happy to rerun/rebase once CI is green.

Refs #378.

No runtime behavior changes.

[codspeed-hq]

Merging this PR will not alter performance

✅ 163 untouched benchmarks

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_{Comparing gcomneno:test/multihosturl-optional-host-378 (a10c291) with main (383eb95)}

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/validators/mod.rs	0.00%	9 Missing ⚠️
src/serializers/mod.rs	0.00%	5 Missing ⚠️

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