Add test mode for Hookdeck destination to skip external API verification

[leggetter]

Description

The Hookdeck destination provider currently requires external API verification for all tokens during destination creation/update. This makes it impossible to write automated tests without real Hookdeck credentials.

7 tests are currently skipped in the SDK test suite due to this limitation.

Current Behavior

When creating or updating a Hookdeck destination, the backend:

1. Parses the token format (base64 encoded source_id:signing_key)
2. Makes an HTTP request to https://events.hookdeck.com/e/{source_id} to verify the token
3. Fails with validation error if the external API call fails

Evidence

Backend Code: internal/destregistry/providers/desthookdeck/desthookdeck.go lines 208-266

func (p *HookdeckProvider) Preprocess(newDestination *models.Destination, originalDestination *models.Destination, opts *destregistry.PreprocessDestinationOpts) error {
    // ...
    if shouldVerify {
        ctx := context.Background()

        // LINE 243: Makes external HTTP request
        sourceResponse, err := VerifyHookdeckToken(p.httpClient, ctx, parsedToken)
        if err != nil {
            return destregistry.NewErrDestinationValidation([]destregistry.ValidationErrorDetail{
                {
                    Field: "credentials.token",
                    Type:  "token_verification_failed",
                },
            })
        }
    }
    return nil
}

Impact

Skipped Tests: spec-sdk-tests/tests/destinations/hookdeck.test.ts

1. should create a Hookdeck destination with valid config (line 61)
2. should create a Hookdeck destination with array of topics (line 66)
3. should create destination with user-provided ID (line 81)
4. Entire describe block: GET /api/v1/{tenant_id}/destinations/{id} (line 167)
5. Entire describe block: GET /api/v1/{tenant_id}/destinations (line 210)
6. Entire describe block: PATCH /api/v1/{tenant_id}/destinations/{id} (line 236)
7. Entire describe block: DELETE /api/v1/{tenant_id}/destinations/{id} (line 304)

Proposed Solution

Add a test mode or configuration flag that skips external token verification:

Option 1: Environment Variable

if os.Getenv("OUTPOST_TEST_MODE") == "true" {
    // Skip external verification in tests
    return nil
}

Option 2: Configuration Flag

hookdeck:
  skip_token_verification: true # Only for testing

Expected Behavior

• In production: Continue to verify tokens against Hookdeck API
• In test mode: Skip external verification, only validate token format
• Tests can create/update/delete Hookdeck destinations without real credentials

Additional Context

See detailed analysis in spec-sdk-tests/TEST_STATUS.md and spec-sdk-tests/GITHUB_ISSUES.md

[leggetter]

@alexluong - also noticed as part of #521

Do you agree with the identified behavior above? What do you think the correct approach is for the tests?