fix(asv3): render finalised messages on mount in MarkdownBlock

[Luis85]

Summary

• The native-renderer branch added by PR feat(agent): MarkdownRenderPort backed by Obsidian native renderer (PR-ASV-7-obsidian) #377 used a single watch(() => props.text, ..., { immediate: true, flush: 'post' }) to drive every render.
• Inside Obsidian's view lifecycle with Vue 3.5, the immediate run fires before the template ref is bound — nativeContainer.value is still null, so rerenderNative() returns early.
• For completed message bubbles whose text never changes again after mount, that early-return is terminal: the body stays empty even though the rendered scratch div is fully populated for streaming bubbles.
• Streaming bubbles hid this because their text keeps growing post-mount, so subsequent watcher firings end up rendering after the container has bound.
• Fix: add an explicit onMounted(() => void rerenderNative()) so finalised messages get one post-mount render even when their text never changes.

How it was caught

• User reported empty user and assistant message bodies in the Obsidian Agent sidepanel; streaming bubble showed text but final bubbles were empty.
• Instrumentation in MarkdownBlock.vue confirmed: the watch's immediate run reported "no el" before mount; the final-message instances mounted (onMounted fired, container bound) but no second watcher firing followed, since text is immutable post-finalisation.
• Streaming bubble (whose text grows) re-fired the watcher and rendered correctly — masking the bug.

Test plan

• npm run typecheck — clean (0 errors)
• npm run lint — 0 errors
• npm run test — 1863 / 1863 passing
• npm run build — clean bundle
• Manual: deploy to test vault, reload plugin in Obsidian, send a fresh prompt — user message body and assistant response body now render their markdown content as expected.

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