Add structured transcription results

[wskr00]

Summary

• Add AudioTranscriptionResult, AudioTranscriptionSegment, and AudioTranscriptionWord to expose structured transcription data.
• Preserve language, duration, segments, words, and speaker annotations from OpenAI verbose and diarized responses.
• Preserve diarized segment data for streaming responses.
• Update the transcription reference documentation.
• Close audio resource input streams and simplify redundant option checks.

Breaking Change

AudioTranscription.getOutput() now returns AudioTranscriptionResult instead of String.

Use .text() when only the transcription text is needed:

String text = response.getResult().getOutput().text();

Testing

• ./mvnw process-sources
• ./mvnw javadoc:javadoc
• ./mvnw clean package
• ./mvnw -pl spring-ai-docs antora

Closes #6640

[wskr00]

Converted this PR to draft because the OpenAI Java SDK currently treats diarized_json as a string response and wraps the raw JSON in Transcription.text.

I've found a upstream issue and a PR. Once the SDK issue is resolved, I will update this PR accordingly.