fix(tts): key the TTS audio cache by a hash of voice + text

[aniketkrs]

Impact

With "Cache TTS" enabled, clicking read-aloud on a message plays the audio of a different message whenever both messages start with a Markdown heading. Markdown answers with headings are the common case for an assistant, so this breaks the feature exactly where it matters most.

Cause

In client/src/hooks/Input/useTextToSpeechExternal.ts, audio is stored with cache.put(new Request(inputText), response) and looked up with caches.match(text) — the raw message text doubles as the cache key. The Cache API parses that string as a URL against the document base (<base href="/">) and ignores the fragment when matching. A message starting with # therefore keys to the site base URL whatever its content, and every such message shares one cache entry: the first audio stored is served for all of them. Text with # later (a heading after the first paragraph) collides with any other text sharing the same prefix up to that #; ? or : early in the text mis-parse as query or scheme.

Verified with a Node repro of the Cache API's key derivation (new URL(text, base) with fragment stripped): two distinct messages starting with ## R..ponse courte both key to https://chat.example/ — collision confirmed.

Fix

Key the cache by a hash of voice + text (getTTSCacheKey, FNV-1a) instead of the raw text, at both the cache.put and caches.match sites. The same repro shows the two messages now key to distinct entries (tts-audio-a8f33905 vs tts-audio-7601e7f4), the key is stable for the same text + voice, and it changes with the voice so switching voices does not serve stale audio. The key is a plain relative path, so it resolves cleanly as a Request URL.

Fixes #16879.

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[github-actions]

👋 Thanks for the contribution! LibreChat merges all changes into dev first — main only moves at release time — so this pull request's base branch was switched from main to dev automatically.

Nothing is needed from you; your commits, reviews and discussion are unchanged. If the diff now shows files you did not touch, rebase onto dev:

git remote add upstream https://github.com/LibreChat-AI/LibreChat.git
git fetch upstream dev
git rebase upstream/dev
git push --force-with-lease

Maintainers: apply the target: main label and restore the base branch if this one genuinely belongs on main.