Allow UrlConnectionHttpClient to retry stream-open IOExceptions and NPEs

[davidh44]

Motivation and Context

Follow-up to #6894. Bare NPEs from HttpURLConnection.getOutputStream() and getInputStream() escaped as runtime exceptions. Wrapped NPEs and checked IOExceptions became UncheckedIOException. RetryableStage catches neither form, so the retry policy never evaluated these failures.

Modifications

• Preserve checked IOExceptions from stream accessors and response-code checks on these paths.
• Convert bare NPEs and runtime exceptions directly caused by an NPE from stream accessors to checked IOException, retaining the cause and logging at DEBUG.
• Catch ProtocolException directly for the Expect: 100-continue workaround. The final-response path retains its existing UncheckedIOException because retrying a completed request could duplicate side effects without recovering the unreadable payload.
• Update javadoc TODO with the known disconnect() race.

Behavior Changes

• Stream-accessor and response-content-check failures can now be evaluated for retry.
• NPE conversion also applies to caller-supplied HttpURLConnection implementations created through create(UrlConnectionFactory).

Testing

• Added unit tests
• Existing tests pass

Types of Changes

• Bug fix (no API surface change, with intentional retry behavior changes described above)
• New feature

[github-actions]

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