Changes

• Fixes a race condition where <style> tags from propagating components (e.g. content collection Content with propagation: 'self') were silently dropped from the build output when await appeared in the template/markup section of an .astro file. Moving the await to the frontmatter was the only workaround.
• The root cause: AstroComponentInstance pre-renders slots eagerly to trigger propagator registration, but when a slot function is async it returns a Promise. bufferPropagatedHead ran before the Promise resolved, collecting zero propagators and dropping their styles.
• Fix: async slot pre-render Promises are tracked on a new pendingSlotEvaluations field of SSRMetadata. bufferPropagatedHead awaits them all before collecting head parts.

Closes #17218

Testing

• Added 'awaits pending async slot evaluations before collecting head parts' to packages/astro/test/units/render/head-propagation/runtime.test.ts. It pushes a delayed Promise that registers a propagator, then verifies bufferPropagatedHead waits for it and collects the resulting style tag.
• Updated runtime-adapters.test.ts and server-islands-render.test.ts fixtures to include the new pendingSlotEvaluations field on SSRMetadata.

Docs

No docs update needed — this is a bug fix restoring already-documented behavior (styles from components should always be collected regardless of where await appears).