fix(elixir-client): Sync CDN-resilience fixes from the TypeScript client

[magnetised]

The TS client has three behaviours around 409 (must-refetch) handling that the Elixir client was missing. All three protect against misbehaving proxies/CDNs that strip query params, cache 409s, or omit response bodies.

Changes

1. Cache-buster on every 409 — adds a fresh cache-buster query param after every must-refetch response, so a CDN that ignores expired_handle in its cache key can't pin us to a stale 409. (poll.ex handle_must_refetch/4)
2. Self-heal stuck expired_handle cache — when stale retries are exhausted, clear the cached expired handle and retry once without it before raising. Recovers from the case where a stale expired_handle value itself is what's poisoning the
   CDN cache slot. (expired_shapes_cache.ex clear_handle/1, poll.ex handle_stale_response/2, shape_state.ex self_heal_attempted? flag)
3. Synthetic must-refetch control message — always emit a must_refetch message on 409, regardless of body content, so subscribers reliably get the signal to clear local state even when a proxy strips the body.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.26%. Comparing base (119731e) to head (d34dd37).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/elixir-client/lib/electric/client/poll.ex	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4231      +/-   ##
==========================================
+ Coverage   64.31%   65.26%   +0.94%     
==========================================
  Files         143      170      +27     
  Lines       19092    20232    +1140     
  Branches     4679     4713      +34     
==========================================
+ Hits        12279    13204     +925     
- Misses       6810     7025     +215     
  Partials        3        3              

Flag	Coverage Δ
elixir	78.84% <92.85%> (?)
elixir-client	78.84% <92.85%> (?)
packages/agents	53.73% <ø> (ø)
packages/agents-runtime	78.61% <ø> (+0.12%)	⬆️
packages/agents-server	66.25% <ø> (+0.21%)	⬆️
packages/agents-server-ui	0.00% <ø> (ø)
packages/electric-ax	33.44% <ø> (+3.33%)	⬆️
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	94.36% <ø> (+0.05%)	⬆️
packages/y-electric	56.05% <ø> (ø)
typescript	64.52% <ø> (+0.21%)	⬆️
unit-tests	65.26% <92.85%> (+0.94%)	⬆️

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[robacourt]

Nice!

[github-actions]

This PR has been released! 🚀

The following packages include changes from this PR:

• @core/elixir-client@0.10.1

Thanks for contributing to Electric!