Enforce write interest registration state

[sainadh777]

Summary

• Complete TODO 17.1 by rejecting duplicate write-interest registration with a localized IllegalStateException until the pending callback begins.
• Clear the registration when OPEN_WRITE processing starts, or when blocking and vectored write continuations consume poller readiness.
• Add focused state-transition tests, update the Coyote changelog, and remove the completed TODO.

Validation

• ant validate
• ant test -Dtest.entry=org.apache.tomcat.util.net.TestSocketWrapperBase (2 tests)
• ant test -Dtest.entry=org.apache.catalina.nonblocking.TestNonBlockingAPI -Dtest.entry.methods=testNonBlockingWrite,testNonBlockingWriteWithKeepAlive (2 tests)
• ant clean && ant
• Full unfiltered ant test: 656 suites, 41,398 tests, 0 failures, 0 errors, 328 skips
• Built distribution smoke: HTTP 200 from /, clean shutdown and no remaining listeners

Draft for maintainer review.

[sainadh777]

@markt-asf @rmaucher Could you review the write-interest state transition and the poller callback clearing points when convenient? This implements TODO 17.1; the full suite and runtime smoke are green. Thank you.