Fix libxev poller callback leaks and re-enable poller tests

[heartwilltell]

Motivation

• Address spurious hangs and dropped wakeups in the libxev-backed poller that caused test_poller cases to be gated (tracked in issues run_xev adapter: multi-fd poll drops second fd's callback #424 and run_xev adapter: inter-test state leak between sequential polls #426).
• Prevent stale libxev completion/callback state from leaking across fd lifecycles and ensure multiple ready fds are drained in a single tick so the scheduler wakes all waiting Gs.

Description

• Add per-fd generation tracking and a CompletionContext userdata in src/runtime/run_xev_bridge.zig so completion callbacks validate the slot generation and ignore stale callbacks from prior fd lifecycles (prevents reuse-related races).
• Stop reusing raw slot pointers as completion userdata and pass &slot.read_ctx / &slot.write_ctx to file.poll / write completions, with the callbacks checking ctx.generation before acting.
• Improve run_xev_close_fd by guarding against double-close and draining cancel progress with several bounded non-blocking ticks before clearing slot state to avoid leaving the loop with dangling references.
• Change run_xev_tick to perform bounded draining of loop.run(.no_wait) until no additional callbacks fire in that tick to avoid dropping additional ready fds (fixes multi-fd callback drop).
• Re-enable the previously gated runtime poller tests in src/runtime/tests/test_poller.c (test_poller_close_while_waiting, test_poller_pipe_read, test_poller_write_park, test_poller_multiple_fds) so the regressions are covered going forward.

Testing

• Attempted to run the runtime test suite with zig build test-runtime, but the zig tool was not available in this environment so the runtime tests could not be executed (no automated tests ran here).
• Verified source changes via repository inspections (git diff) and updated test files to re-enable the gated tests; runtime tests should be executed in CI or a developer environment with zig installed to validate the fixes.

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Codex Task

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	e00efbc	Commit Preview URL Branch Preview URL	Apr 27 2026, 09:58 PM