tidy URL validator implementations

[davidhewitt]

Change Summary

Reading through the logic for URL validation ahead of implementing my shot at #1719, I wanted to clean up a couple things:

• Seemed always better to check for URL types before attempting to extract strings
• Some conflation of "scheme" and "schema", which I wanted to clean up for my own sanity
• I added some test cases to demonstrate that the trailing slash is only added to an "empty" path.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1788 will not alter performance

_{Comparing dh/tidy-url-validation (79dfad7) with main (1cd5ee3)}

Summary

✅ 161 untouched
🆕 2 new

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	test_multi_host_url_core	N/A	33.5 µs	N/A
🆕	test_url_core	N/A	25 µs	N/A

[davidhewitt]

Will merge as a trivial clean-up