Fix ArgumentError when a host LiveView uses stream/3 by dropping reserved assigns at every field-component spread site

[Flo0807]

Summary

Backpex spreads the parent LiveView's assigns into child field LiveComponents after dropping a hand-maintained list of keys. That list was missing :streams everywhere, and :uploads / :myself at three of the six sites. When a host LiveView calls stream(socket, :name, []) (or allow_upload/3), the reserved key flows into the child and Phoenix.Component.assign/3 raises:

** (ArgumentError) :streams is a reserved assign by LiveView and it cannot be set directly
    (phoenix_live_view 1.1.30) lib/phoenix_component.ex:1467: Phoenix.Component.validate_assign_key!/1
    (phoenix_live_view 1.1.30) lib/phoenix_component.ex:1405: Phoenix.Component.assign/3
    (stdlib 7.3) maps.erl:894: :maps.fold_1/4
    (backpex 0.18.x) lib/backpex/fields/belongs_to.ex:77: Backpex.Fields.BelongsTo.update/2

This affects every field whose update/2 does socket |> assign(assigns): BelongsTo, HasMany, HasManyThrough, MultiSelect, InlineCRUD.

Changes

• Add Backpex.HTML.Resource.lv_reserved_assigns/0 returning [:flash, :uploads, :streams, :socket, :myself] — the full set rejected by Phoenix.Component.assign/3 (plus :flash, which was already dropped at every site).
• Route all six Map.drop(assigns, ...) spread sites through the new helper:
  • lib/backpex/html/resource.ex (3 sites: live_resource_field/1, inlined_resource_field/1, resource_form_field/1)
  • lib/backpex/html/item_action.ex (1 site)
  • lib/backpex/html/resource/resource_form_main.html.heex (1 site)
  • lib/backpex/html/resource/resource_index.html.heex (1 site)
    Site-specific extras (:fields) are prepended with [:fields | lv_reserved_assigns()].
• Add test/backpex/html/resource_test.exs asserting the helper returns the full reserved set and that dropping it removes :streams from a parent-style assigns map.

No version bump in this PR — will be handled separately when the release is cut.

Test plan

• mix format
• mix lint (compile / credo / 95 doctests + 166 tests, 0 failures)
• mix test test/backpex/html/resource_test.exs — 2/2 pass
• Demo smoke test in browser: Posts index, Posts /new, Users index, Send-email resource action modal — all render without server or console errors
• Verify in a consumer app that calls stream/3 on mount and renders a BelongsTo field — page renders without ArgumentError

[Copilot]

Pull request overview

Fixes a crash when a host LiveView uses stream/3 (or uploads) by ensuring Backpex never forwards LiveView-reserved assigns (e.g. :streams, :uploads, :myself) when spreading parent assigns into child LiveComponents.

Changes:

• Centralizes the LiveView-reserved assign keys in Backpex.HTML.Resource.lv_reserved_assigns/0.
• Updates all documented assign-spread sites to drop the centralized reserved set (plus site-specific keys like :fields).
• Adds tests verifying the reserved set includes :streams and that dropping it removes :streams from a parent-style assigns map.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/backpex/html/resource.ex	Adds lv_reserved_assigns/0 and routes multiple LiveComponent assign spreads through it.
lib/backpex/html/item_action.ex	Drops the centralized reserved assigns (plus :fields) when spreading assigns into the modal form component.
lib/backpex/html/resource/resource_index.html.heex	Drops reserved assigns (plus :fields) when spreading assigns into the resource-action modal component.
lib/backpex/html/resource/resource_form_main.html.heex	Drops reserved assigns when spreading assigns into the main form component.
test/backpex/html/resource_test.exs	Adds coverage for the reserved-assign helper and for dropping :streams.

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