fix: Propagate IOExceptions in OkHttpFluxRequestBody

[mlichtblau]

Description

Since Exceptions.propagate(e) in OkHttpFluxRequestBody wraps checked IOExceptions in a runtime exception, the original exception is not propagated correctly to okhttp causing missed retries and logging noise.

fixes #47169

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Thank you for your contribution @mlichtblau! We will review the pull request and get back to you soon.

[Copilot]

Pull Request Overview

This PR adds exception unwrapping logic to properly propagate IOException instances that are wrapped in RuntimeException during the request body writing process.

• Wraps the blocking call to requestSendMono.block() in a try-catch block to handle exceptions
• Unwraps and re-throws IOException when it's the cause of a caught RuntimeException
• Ensures IOException is properly propagated to OkHttp as expected by the writeTo method signature

[mlichtblau]

@microsoft-github-policy-service agree

[alzimmermsft]

Thanks for the fix @mlichtblau !