#25193 [swift6] Encode request bodies as compact JSON

Fixes #25186.

Drop .prettyPrinted from CodableHelper's default JSONEncoder, so request bodies are compact JSON. The JSON content is identical, only the whitespace goes. A jsonEncoder set by the user is unaffected, and so is anyone who sets outputFormatting themselves.

DateFormatTests.testCodableAlwaysResultsInUTCEncodedDate already removes .prettyPrinted, so that line becomes a no-op and the tests are unaffected.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/swift6*.yaml || exit
  

  Commit all changed files.
• @mention the technical committee: @jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

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Summary by cubic

Makes Swift 6 request bodies compact JSON by removing .prettyPrinted from CodableHelper's default JSONEncoder, so outgoing bodies no longer carry unnecessary whitespace.

• Request bodies are now smaller on the wire; the JSON content is unchanged.
• A user-supplied jsonEncoder is unaffected, and anyone setting outputFormatting themselves keeps full control.
• Regenerates all swift6 samples to match the new default.