Normalise the strategy's wait time to int before jitter

[noahott]

Fixes #22.

A strategy may be any callable, and nothing requires it to return an int. cap() is type-preserving, so a float-returning strategy reaches mt_rand() unchanged — and mt_rand() takes int parameters.

Three symptoms, all from the same line:

Strategy returns	Jitter	Today
15.667	on	Deprecated: Implicit conversion from float 15.667 to int loses precision (PHP 8.1+)
15.667	off	returns float(15.667), contradicting getWaitTime()'s @return int
1e20	on	TypeError: mt_rand(): Argument #2 ($max) must be of type int, float given

The last one is a hard crash, and it is reachable whenever no wait cap is set — cap() returns the strategy value untouched in that case, since is_int(null) is false.

The bundled strategies all cast to int already, which is why none of this shows up in normal use.

The change

-        return $this->jitter($this->cap($waitTime));
+        return $this->jitter((int) min(PHP_INT_MAX, $this->cap($waitTime)));

Two deliberate details:

• After cap(), not before, so the negative-overflow guard added in Added protection against overflows in the wait time calculation. #20 still runs first. testWaitCapOverflow still passes.
• min(PHP_INT_MAX, ...) rather than a bare cast. Casting a float above PHP_INT_MAX is undefined in PHP; clamping first makes the 1e20 case defined instead of platform-dependent. If you'd rather keep the diff minimal, (int) $this->cap($waitTime) fixes the deprecation and the TypeError too — it just leaves the out-of-range value undefined rather than clamped. Happy to switch.

Casting before jitter() rather than after also means the non-jittered path returns an int, matching the docblock.

Tests

Three added to BackoffTest, following the existing style (no attributes, $this->assertX()):

• testFloatReturningStrategyProducesIntegerWaitTime — jitter off, asserts assertIsInt(). This is the one that needs no error-handler juggling, so it's the durable regression test.
• testFloatReturningStrategyDoesNotTriggerDeprecationUnderJitter — installs a temporary E_DEPRECATED handler, restores it in a finally. Worth noting phpunit.xml sets neither failOnDeprecation nor failOnWarning, so without an explicit handler a test here cannot observe the deprecation at all.
• testFloatStrategyBeyondIntMaxDoesNotThrow — the TypeError case.

Each fails on master and passes with the patch. On baab60f + tests only:

1) testFloatReturningStrategyProducesIntegerWaitTime
   getWaitTime() is documented @return int
   Failed asserting that 15.667 is of type int.

2) testFloatReturningStrategyDoesNotTriggerDeprecationUnderJitter
   Failed asserting that two arrays are identical.
   +    0 => 'Implicit conversion from float 15.667 to int loses precision'

3) testFloatStrategyBeyondIntMaxDoesNotThrow
   TypeError: mt_rand(): Argument #2 ($max) must be of type int, float given

With the patch:

PHPUnit 10.5.64 by Sebastian Bergmann and contributors.
Runtime:       PHP 8.3.31

OK (37 tests, 89 assertions)

Verified on PHP 8.3.31. Your CI matrix is PHP 8.2, where the deprecation also applies (8.1+).