feat: custom priority validator

[basgys]

This PR allows configuration of how Bulwark handles out-of-range priority values. Three built-in options cover the common use cases:

• ClampInvalidPriority (default) — silently clamps the priority to the lowest valid level and emits a warning log. Safe for production: a misconfigured or user-supplied priority value won't crash the
  process.
• RejectInvalidPriority — returns an error, letting the caller decide how to handle it. Suited for APIs where the caller owns priority validation.
• AssertValidPriority — panics with a descriptive message. Useful in tests or staging to catch programming errors early.

The previous behaviour was an implicit runtime panic (index out of bounds) whenever an invalid priority was passed to Throttle or WithAdaptiveThrottle. Since priority values can come from configuration or user input, crashing the process is a disproportionate response — hence the safer clamp default.

The custom validator can be set via WithPriorityValidator(fn).

---

Note

Medium Risk
Changes how invalid priorities are handled across all throttle entrypoints (clamp/log vs error/panic), which can affect runtime behavior and error paths for callers.

Overview
Adaptive throttling now validates request priority values before indexing counters, avoiding the previous out-of-bounds panic behavior.

NewAdaptiveThrottle stores the configured priority count, defaults to a safe ClampInvalidPriority validator, and rejects priorities <= 0. Throttle, generic Throttle[T], and WithAdaptiveThrottle[T] now run the validator and may return an error before invoking the throttled function.

Adds WithPriorityValidator plus built-in validators (AssertValidPriority, ClampInvalidPriority with warning log, and RejectInvalidPriority) with accompanying tests.

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