Validate form encoding before replacing request body state

[OskarEichler]

Summary

Validate the enctype before replacing body/form state. Keep the assignments inside the accepted branch, preserving valid-form behavior and return value.

Reproduction

With req = Net::HTTP::Post.new('/'); req.body = 'keep', call req.set_form([['name', 'value']], 'wrong') and rescue ArgumentError. Before, req.body becomes nil; after, it remains keep. The same problem affects existing body_stream and multipart form state.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 201 tests, 914 assertions, zero failures/errors, both baseline and this isolated patch.
• 8 focused checks cover invalid enctype errors and preserved string/stream/form state, plus valid replacement and Content-Type behavior.
• Ruby syntax and git diff --check pass. Supplemental Lint scan retains the same 25 pre-existing findings; no new findings by file/cop.
• No test/spec files were added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions were run externally.

Compatibility and limitations

No API or dependency change. Intentional error-path correction: a rejected enctype no longer discards the existing request body.

Based on master 23e859e92e10e43cad520fcacac3fd43640dba7b. Optional external OpenSSL tests and other Ruby/OS combinations were not run. Existing related issues/PRs were checked; this is a focused correctness change, not a claim of exhaustive coverage.