Limit parse cache growth to avoid memory leak

[benfdking]

Summary

• bound parser cache size and clear when threshold exceeded to prevent unbounded memory use

Testing

• cargo test -p sqruff-lib-core

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https://chatgpt.com/codex/tasks/task_e_68b6a6262c348330bc8c932c28d79bf2

[github-actions]

Benchmark for 5e75f7e

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Test	Base	PR	%
DepthMap::from_parent	61.0±0.53µs	60.3±2.14µs	-1.15%
fix_complex_query	12.2±0.17ms	12.3±0.09ms	+0.82%
fix_superlong	146.2±13.89ms	155.1±18.20ms	+6.09%
parse_complex_query	4.2±0.11µs	4.1±0.05µs	-2.38%
parse_expression_recursion	7.3±0.09µs	7.2±0.11µs	-1.37%
parse_simple_query	1050.2±20.02ns	1055.7±13.37ns	+0.52%

[github-actions]

Benchmark for 11a1324

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Test	Base	PR	%
DepthMap::from_parent	59.6±0.82µs	60.9±0.88µs	+2.18%
fix_complex_query	12.0±0.06ms	12.3±0.06ms	+2.50%
fix_superlong	146.0±15.68ms	152.1±15.92ms	+4.18%
parse_complex_query	4.2±0.05µs	4.1±0.05µs	-2.38%
parse_expression_recursion	7.4±0.10µs	7.2±0.11µs	-2.70%
parse_simple_query	1059.6±16.18ns	1068.9±15.24ns	+0.88%

[github-actions]

Benchmark for a830aa6

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Test	Base	PR	%
DepthMap::from_parent	61.1±0.69µs	60.8±1.50µs	-0.49%
fix_complex_query	12.1±0.12ms	12.3±0.46ms	+1.65%
fix_superlong	140.8±16.92ms	148.9±16.15ms	+5.75%
parse_complex_query	4.2±0.04µs	4.1±0.04µs	-2.38%
parse_expression_recursion	7.2±0.09µs	7.3±0.07µs	+1.39%
parse_simple_query	1077.6±15.14ns	1093.3±20.25ns	+1.46%