fix(mobile): restore the composer message when a send fails (port #3785)

[Gilbert09]

Problem

On the mobile task composer, a failed send silently lost the user's work. handleSend cleared the typed text and attachments and fire-and-forgot the send; when handleSendPrompt rejected (network/API error) it only logged and showed a "Failed to send" alert — the message and any picked attachments were already gone.

Changes

Ports the desktop behaviour from #3766 and #3785 (fix(agent): preserve messages until send succeeds and its follow-up fix: clear sent messages without blocking the composer) to the mobile app.

• The send path now reports success/failure back to the composer: onSend / handleSendPrompt return Promise<boolean> across every branch — queued-message edit, terminal-session resend, queue mode, steer/interrupt, and the normal send.
• The composer still clears optimistically on submit and stays responsive — sending is not blocked and typing is not disabled while a send is in flight.
• On failure the submitted text and attachments are restored into the composer, but only when the composer is still empty (the user hasn't started a new draft) and only when the failed submission is still the latest one (guarded by a monotonically increasing submission id, so a stale failure can't clobber a newer draft).
• The clear/restore decision logic is factored into a small pure helper module (submitComposerMessage.ts) so it is unit-testable without rendering React Native components.
• Existing behaviour is preserved: queued-message in-place editing, the pendingPromptRecoveryStore new-task recovery flow, haptics, keyboard dismissal, and trackPromptSent analytics.

How did you test this?

• Added Vitest coverage for the new pure helpers (submitComposerMessage.test.ts): successful send clears and stays cleared; failed send restores text + attachments; failed send does not restore when a new draft has been typed; a stale failure does not clobber a newer submission; isComposerEmpty cases.
• pnpm --filter @posthog/mobile test — full mobile suite, 494 tests passing.
• Typechecked the changed files and confirmed zero new type errors vs. the base tree.
• biome check clean on the changed files.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 028ca90.}