Add a provider-adapter contract test harness

[brudex]

Proposed labels: testing, area:providers, good first issue

Context

Hype Engine routes provider behavior through its platform-service registry,
but its provider adapters do not share root-level contract coverage. A small
mocked test harness would make adapter expectations explicit without using
real provider accounts or tokens.

Suggested starting points

• services/platform/index.js
• provider adapter modules under services/platform/
• test/ (the repository's existing Node test suite)

Scope

Introduce a focused test helper and contract tests that verify supported
adapters expose testCredentials and publishPost. Cover safe structured
failure behavior for incomplete fixtures, including the Instagram-to-Facebook
alias. Mock all HTTP boundaries.

Non-goals

• Do not add or change provider credentials.
• Do not make real HTTP requests, publish content, or refactor provider
  implementation details beyond the minimum test seam.

Acceptance criteria

• The supported registry adapters satisfy the documented interface.
• Incomplete test fixtures produce a safe, structured failure rather than an
  uncaught exception or secret-bearing log.
• The Instagram alias resolves to the intended Facebook publishing path.
• The new tests make no network calls and pass with npm test.

Verification

1. Run npm test with the new tests included.
2. Confirm the tests use fakes/mocks and do not read environment provider
   tokens.
3. Run git diff --check.