Co-authored-by: albertcht <9117929+albertcht@users.noreply.github.com>

The previous fix imported `Hypervel\Context\Context`, which does not exist
in this codebase — `hypervel/context` only exposes `CoroutineContext`,
`RequestContext`, `ResponseContext`, `ParentCoroutineContext`,
`ReplicableContext`. Calling `guessMimeType()` threw a class-not-found
error.

Switch to `CoroutineContext::getOrSet()` to match how every other ported
package uses per-coroutine caching (JwtGuard, Sentry\Hub, UrlGenerator,
Log\Context\Repository).

Rename the constant to `FINFO_CONTEXT_KEY_PREFIX` per CLAUDE.md naming
convention (existing examples: `CACHED_SCHEME_CONTEXT_KEY`,
`CACHED_ROOT_CONTEXT_KEY`) and promote it to `public` so the regression
test can read the cached `finfo` back out of context.

…tions

- Extend `Hypervel\Tests\TestCase` instead of raw `PHPUnit\Framework\TestCase`
- Drop the `RunTestsInCoroutine` trait (already on the base class)
- Drop `@internal` / `@coversNothing` docblock and `: void` return types
- Update fixture path to `Fixtures/` (capital F)

Replace the bare `Coroutine::create()` regression test with `parallel()`,
which awaits the children and propagates assertion failures back to
PHPUnit. The previous form spawned 5 coroutines without a `WaitGroup`,
so failures inside them never failed the test.

Strengthen the test to actually exercise the per-coroutine isolation
the fix provides: each coroutine pulls its cached `finfo` back out of
`CoroutineContext` via `FINFO_CONTEXT_KEY_PREFIX` and the test asserts
all 5 instances are distinct objects. Reverting to the shared static
cache would now fail this test.