fix(compilers/openapi): reconcile allOf branches that redeclare a field

[OmarAlJarrah]

Summary

Fixes hundreds of spurious pass/duplicate-wire-name errors when compiling
specs whose allOf schemas have inline branches that redeclare the same field.

allOf lowering appended every inline branch's properties into one model. When
two branches declared a property with the same name — a common real-world shape
where a documented object is followed by a doc-stripped duplicate that marks
some fields required — the model ended up with several properties sharing one
wire name. The validate pass then correctly flagged that corrupted model.

On the GitHub REST API spec this produced 304 errors across forkee,
issue, workflow_job, project_card, author, and committer schemas
(e.g. webhook-fork/properties/forkee alone had 168 properties, 78 duplicated).

allOf is an intersection: a redeclared field is one logical property, present
if the instance satisfies every branch and required if any branch requires it.
The fix reconciles same-wire-name properties into a single property (OR-ing
Required, first declaration defining the shape) instead of appending
duplicates. Ref/mixin classification (ir-design §4.3) is unchanged.

Test plan

• New TestAllOf_OverlappingInlineBranchesReconcile — two inline branches
  redeclaring a field reconcile to one property; Required reflects the
  branch that requires it; the documented declaration defines the shape.
• go test ./... — all packages pass (golden snapshots unchanged).
• End-to-end on the GitHub REST spec: 304 errors -> 0; only the pre-existing
  240 benign openapi/degraded-construct info diagnostics remain; exit code 0.