fix: schema-sharded fallback routing (#819)

[Munawwar]

PR logic explained at #819 (comment)

TODO: Haven't added Client/query-engine test yet.

There are two edge cases I haven't handled:

1. this could work incorrectly if the schema is still loading in the background and its not completed yet
2. load_schema = Off (well .. what to do here even?)

@levkk I added more tests to increase coverage and then reverted it. It's just significantly more LoC

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[codecov]

Codecov Report

❌ Patch coverage is 86.88525% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/query/select.rs	52.94%	8 Missing ⚠️
pgdog/src/frontend/router/parser/query/delete.rs	68.18%	7 Missing ⚠️
pgdog/src/frontend/router/parser/query/update.rs	68.18%	7 Missing ⚠️
pgdog/src/frontend/router/parser/query/mod.rs	87.50%	2 Missing ⚠️

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[Munawwar]

@levkk changes are ready for review. Check my previous comment as well please