Fix FilterExpressionTextParser shared error state

[kdkrkwhr]

Overview

FilterExpressionTextParser used a shared singleton DescriptiveErrorListener,
so every parser instance wrote into the same mutable errorMessages list. A
failed parse on one instance could leak or clear error messages observed by
another instance, including under concurrent use.

This scopes the error listener to the parser instance so each one owns its own
error state.

Fixes #6807

Changes

• Construct a per-instance DescriptiveErrorListener instead of the shared INSTANCE.
• Remove the unused DescriptiveErrorListener.INSTANCE constant.
• Add a getErrorMessages() accessor (test-only) to assert instance isolation.
• Add regression tests:
  • testErrorStateIsNotSharedAcrossInstances
  • testParallelParsingDoesNotMixErrorState (8-thread concurrent parse)

Verification

FilterExpressionTextParserTests passes: 17/17, 0 Checkstyle violations.

[qixiangme]

Regarding @ViniciusKoiti's comment on #6807, since the listener is now per-instance, would concurrent parse() calls on the same FilterExpressionTextParser instance still share the same error state?

Would a per-invocation listener make sense for covering that case as well?

If this makes sense, I'd be happy to work on this case.

[kdkrkwhr]

Thanks @qixiangme good catch. You're right.

The per-instance change fixes cross-instance leakage, but a shared instance would still
mix state across concurrent parse() calls. I'll move the listener to a per-invocation local
inside parse() — that covers both cases and drops the clear() call. Will update the PR and
adjust the test to use a single shared instance.

[kdkrkwhr]

Done, moved the listener into parse() (per-invocation, as suggested in the issue discussion) and added a same-instance regression test. Ready for another look.