fix: harden HTTPServer failure handling #2283 #2284

[arnabnandy7]

Summary

Fixes #2283 and #2284 by hardening the standalone HTTP server exporter failure paths.

Changes

• Return a generic HTTP 500 response body for scrape failures instead of exposing Java stack traces.
• Log full scrape exceptions server-side for debugging.
• Replace the default HTTP server executor's zero-capacity queue and blocking rejection handler with a bounded queue and non-blocking rejection.
• Stop draining arbitrary request bodies in the invalid authenticated-subject path.
• Add adapter, server, and integration-test coverage for the updated behavior.

Testing

• mise run lint:fix
• .\\mvnw.cmd clean test -pl prometheus-metrics-exporter-httpserver "-Dtest=HTTPServerTest,HttpExchangeAdapterTest" "-Dcoverage.skip=true" "-Dcheckstyle.skip=true"
• .\\mvnw.cmd clean test-compile -pl integration-tests/it-exporter/it-exporter-test "-Dcoverage.skip=true" "-Dcheckstyle.skip=true"
• .\\mvnw.cmd install -DskipTests "-Dcoverage.skip=true"

[github-actions]

⚠️ API changes detected — maintainer review required

This PR modifies the published API diff for the following module(s):

• prometheus-metrics-exporter-httpserver

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.