[studio][ISSUE #10904] Tolerate malformed Proxy retry counters

[messere1]

What changed

• tolerate malformed reconsume-times values and keep the existing default of 0
• tolerate malformed max-reconsume-times values and leave the optional header field unset
• continue clearing both internal retry properties after handling them
• add a regression test for both malformed counters

Why

ProducerProcessor parsed both retry counters with Integer.valueOf. A malformed stored property raised NumberFormatException while building the broker request header and aborted Proxy message forwarding. The same method already handles a malformed born timestamp defensively.

Impact

Valid retry messages keep their current request-header values. Only malformed counters use safe defaults; no public API changes are introduced.

Fixes #10904

Validation

• mvn -pl proxy -Dtest=ProducerProcessorTest -Djacoco.skip=true test
  • 7 tests, 0 failures, 0 errors
  • Checkstyle: 0 violations
  • SpotBugs: 0 issues

JaCoCo is skipped locally because the repository's JaCoCo 0.8.5 agent does not support Java 17 class files.