fix(flags): align local string matching with the flags service

[marandaneto]

💡 Motivation and Context

The posthog-server local evaluator could disagree with the released /flags service for exact matching, JSON values, and case-insensitive string operators.

This fixes PostHog/posthog#78019 by matching the current Rust backend contract:

• exact and is_not apply the backend's boolean-like filter gate before ordinary array membership.
• Boolean-only and empty filter arrays preserve the backend's aggregate truthiness behavior.
• Composite values use compact JSON with recursively sorted keys and backend-compatible escaping.
• Integer values retain their JSON spelling.
• Floating-point matching falls back to the server because JVM formatting is not identical to serde_json for every finite double.
• Exact comparisons use full Unicode lowercase, including final sigma and dotted İ.
• Contains, prefix, and suffix comparisons retain ASCII-only lowercase.

The proposed backend cleanup in PostHog/posthog#90694 is a separate draft. This PR intentionally follows the released backend until that proposal is accepted.

The patch changes only posthog-server and includes a patch changeset.

💚 How did you test it?

Red-green regression evidence:

• Boolean coercion initially failed for false versus "banana"; the focused evaluator suite now passes.
• Composite stringification initially failed for "[1,2]" versus [1, 2]; the focused evaluator suite now passes.
• Review follow-ups reproduced null-member omission, broad long-s boolean matching, and nested non-finite failures before their fixes.

Validation:

• ./gradlew :posthog-server:test --tests 'com.posthog.server.internal.FlagEvaluatorTest' --no-daemon
• make checkFormat
• ./gradlew :posthog-server:animalsnifferMain :posthog-server:animalsnifferTest --no-daemon
• $HOME/.pi/agent/skills/autoreview/scripts/autoreview --mode local

The full :posthog-server:test suite reached 512 passing tests. Five unrelated Mockito inline-agent tests fail on the local JDK. Required CI remains the authoritative full-suite check.

Autoreview findings about operand direction and whole-array matching were rejected after verification: propertyValue is the filter operand, and the focused 100-test evaluator class passes the cited vectors.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Pi implemented and reviewed the change under human direction using the address-pr-comments, check-pr, autoreview, and karpathy-guidelines skills. The released Rust matcher was the source of truth; the separate backend draft was not adopted.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(flags): align local string matching ..." | Re-trigger Greptile}

[arnohillen]

computeExactMatch (posthog-server/src/main/java/com/posthog/server/internal/FlagEvaluator.kt:238) still skips the service's boolean-coercion gate (is_truthy_or_falsy_property_value, rust/feature-flags/src/properties/property_matching.rs:212-220), so exact with filter ["false"] and property "pro" returns false locally and true on /flags (same for filter false vs "banana" and filter [] vs true), while filter ["true","false"] vs "true" returns true locally and false on /flags; pre-existing (identical results on main) and posthog-python's compute_exact_match has the same gap, so follow-up rather than a blocker here.

[marandaneto]

Addressed the released-backend parity findings in 41fa02a. A focused red test reproduced false versus "banana" returning false locally before the fix; the evaluator now applies the backend boolean-array gate first, including empty-array truthiness, and the same suite passes. Composite values now use compact recursively sorted JSON, while floating-point values return inconclusive and fall back to /flags because JVM formatting cannot reproduce every serde_json spelling. Strong final-sigma and dotted-I vectors pass without changing Kotlin lowercase. PostHog/posthog#90694 remains the separate proposed backend cleanup.