[followup] Hoist docs-parity AST walker to internal/testhelpers/docsparity/

[trilamsr]

Source: docs/followups/opportunistic.md row 13 (Next up).

The AST walker in components/receivers/dcgm/docs_parity_test.go (RUNBOOK ↔ IncError-kind parity, README ↔ example_config defaults) is the bar every new receiver should clear, but it's currently ~300 LOC inside dcgm. Moving it to internal/testhelpers/docsparity/ makes the bar invisible-to-explicit and saves the duplication on the second receiver author.

Trigger status (FIRED): kernelevents already maintains a parallel implementation at runbook_kinds_test.go — second caller exists today.

Files:

• components/receivers/dcgm/docs_parity_test.go — source.
• components/receivers/kernelevents/runbook_kinds_test.go — second caller (consolidate w/ source).
• New: internal/testhelpers/docsparity/docsparity.go — exported API like docsparity.AssertParity(t, ReceiverPath, RunbookPath).

Coordinate with #137 (the AST walker resolveIncErrorCall CallExpr fix) — implement both in one refactor so the hoist captures the CallExpr handling.

Acceptance:

• Both dcgm and kernelevents tests call the hoisted helper.
• New receiver authors invoke it in a single line; no AST boilerplate to copy.

[trilamsr]

Outdated by RFC-0013 distribution-first pivot (docs/rfcs/0013-distro-first-pivot.md §7). The referenced receiver/scaffold is deleted in v0.1.0/v0.2.0; this followup no longer applies.