test(policydsl): add regression tests for stack-overflow DoS (#5304)

[PrazwalR]

Adds regression tests for #5304 (stack overflow via deeply nested
policy DSL expressions).

Why

FromString() previously used Knetic/govaluate, whose recursive
planner had no depth/size limit and could overflow the goroutine
stack on maliciously nested AND/OR/OutOf policy strings
(confirmed: reproduces fatal error: stack overflow in
planValue -> planAccessor -> planFunction -> planPrecedenceLevel
on the old dependency).

This was already fixed on main by the govaluate -> expr-lang/expr
migration in #5480 (common/policydsl/policyparser.go), which
enforces expr's DefaultMaxNodes (1e4) at parse time. This PR adds
no production code change — only tests pinning that behavior so it
cannot silently regress:

• deeply nested (up to 200k levels) AND/OR policies parse without
  crashing, either succeeding or failing with the expected
  "exceeds maximum allowed nodes" error
• the node cap is confirmed structural (AST size), not a naive
  input-length check — a wide-but-shallow 2000-arg OutOf still parses
• malformed/adversarial input (unbalanced parens, 1MB tokens, unknown
  gates) never panics

Test plan

• go test -race -v ./common/policydsl/... — all pass
• go vet ./common/policydsl/... — clean
• gofmt -l common/policydsl/policyparser_deepnest_test.go — clean

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