fix: prevent RuntimeError missing Absinthe.Phase.Document.Result for custom :result_phase (OPS-3643)

[palantir-valiot]

Summary

Add support for a :result_phase option to Absinthe.Plug (and a post-pipeline normalization pass) so that replacing Absinthe.Phase.Document.Result (either via the new option or a user :pipeline override) no longer produces RuntimeError: Could not find phase Elixir.Absinthe.Phase.Document.Result when Absinthe phases jump to the result phase on validation/complexity/parse/etc. errors.

The root cause was a mismatch: for_document (and carried options on early phases) always targeted the default Result, while the pipeline list contained a replacement; jumps then did Pipeline.from(remaining, Result) against a list that no longer contained it.

• TDD: added tests (custom pipeline replace + direct option) exercising the complexity-error path that triggers the jump; they went red with the exact stacktrace, then green.
• Native support: result_phase is now a first-class raw/init option, passed through, replaced in default_pipeline/2, and the carried options updated.
• Defensive: Absinthe.Plug.ensure_result_phase_consistent/1 (invoked after document-provider pipeline munging) rewrites :result_phase in any carried opts to whatever *Result phase is actually present in the final list. This makes even naive default_pipeline(...) |> Pipeline.replace(Result, MyOrdered) patterns safe.
• Also upgraded plug/poison (and transitives) + loosened plug constraint (per "keep dependencies fresh" rule) in same PR.

Files changed: mix.exs, mix.lock, CHANGELOG.md, lib/absinthe/plug.ex:127 (raw_options + docs + default_pipeline + helpers), lib/absinthe/plug/request/query.ex:80 (the ensure call), test/lib/absinthe/plug_test.exs (TDD tests + CustomResult helper + pipeline fn).

Why

OPS-3643 (stitchex-lamosa-gto-prod prod crash, high severity code bug). The error was reported against valiot/absinthe_plug; the trigger was stitchex's StitchexWeb.Plugs.GraphQL.pipeline/2 (and similar patterns elsewhere) that do Absinthe.Plug.default_pipeline(...) |> Absinthe.Pipeline.replace(Absinthe.Phase.Document.Result, OrderedResult) for field-order preservation. Same pattern is used by other consumers for Phoenix result structs etc. The fix belongs in the plug.

See triage: NOTIFY+FIX for identifiable RuntimeError in first-party absinthe_plug.

Test plan

• Added TDD test first; reproduced exact ** (RuntimeError) Could not find phase Elixir.Absinthe.Phase.Document.Result at plug.ex:run_query + pipeline.from (using @complex_query + max_complexity to force Complexity.Result jump).
• MIX_ENV=test elixir ... -S mix test test/lib/absinthe/plug_test.exs (with pa for test-only codec) → 30/30 green after fix (including original complexity, new custom-pipeline, and new direct-result_phase cases).
• mix format mix.exs lib/ test/
• mix hex.outdated + mix deps.update (plug/poison) + constraint bump in mix.exs; tests still green post-upgrade.
• git diff inspected (no debug, no scope creep, no empty hunks, only the described files).
• Manual verification that both result_phase: MyMod in init/1 and post-default replace now result in the custom phase being present in the pipeline list and in all carried phase options.
• No other valiot/ repo change needed (the crash site was in absinthe_plug; stitchex will just work, and can later simplify).

Closes OPS-3643

[linear-code]

OPS-3643