[C++] Reset ParseJson field stack between documents

[Parcifalix00]

Summary

Parser::ParseJson() clears builder_ before parsing a new JSON document, but
does not clear field_stack_.

If malformed JSON causes vector parsing to return early, temporary entries can
remain in field_stack_. Reusing the same Parser for subsequent independent
ParseJson() calls can therefore accumulate parser state across documents.

This change clears field_stack_ at the same independent-document boundary as
builder_.

Regression test

The regression test reuses one Parser across malformed JSON vector parses and
verifies that temporary field_stack_ entries do not accumulate across calls.
It also verifies that a subsequent valid JSON document leaves the stack empty.

The identical test fails on the previous implementation and passes with this
change.

Impact

Long-lived integrations that recover from JSON parse errors and reuse a
Parser can otherwise retain temporary state across independent inputs. With
repeated malformed inputs this can produce cumulative process-memory growth and,
under a finite memory budget, eventual resource exhaustion.

This change addresses parser-state retention and availability/resource
exhaustion. It does not claim memory corruption, confidentiality or integrity
impact, privilege escalation, or code execution.

Validation

• Deterministic regression: fails before the fix and passes after it.
• Configured CTest suite: passes.
• git diff --check: clean.
• C++ formatting validation: passes.

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