Use converted strings in query and fragment setters

[OskarEichler]

Summary

Use and copy the result of to_str in both query= and fragment=. The current code calls to_str but then continues invoking String methods on the original non-String receiver.

Reproduction

Give an Object a to_str method returning 'a b', and assign it to the query or fragment of URI('https://example.test/a'). Before, delete! raises NoMethodError on the Object; after, the component is encoded like an ordinary String. Copying also protects a shared or frozen conversion result.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 93 tests / 3,039 assertions / zero failures or errors, both baseline and this isolated patch.
• 42 checks cover both setters, mutable/frozen conversion results, whitespace removal, percent encoding, Unicode, empty strings, unchanged source values and clearing with nil.
• Syntax and git diff --check pass. Supplemental Lint adds no findings against the 31-finding baseline.
• No test/spec files added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions ran externally.

Compatibility and limitations

String-convertible values now work. Direct String and nil behavior is preserved; there is no new coercion through to_s. No public API removal, dependency change or Ruby minimum change.

Based on master 696df43b0be4c05d3a0dcf7db582126c915d860d. Other Ruby/OS runtimes were not executed locally. No production traffic or live mail/FTP services were used. Existing related PRs/issues were checked; this is a focused contribution, not exhaustive behavioral coverage.