ci: standardize custom-code budget reporting (#4018)

Aligns the existing custom-code budget tooling with the shared generated
templates, including generated-file headers, documentation, and
publisher test-fixture formatting.

The existing API reference, runtime behavior, public API, and
repository-owned budget policy remain unchanged.

Validation: SDK formatting and all 59 offline custom-code tests passed
locally, including the compiler hash contract.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>