[INS-465] Add datadogapikey detector to defaults.go

[mustansir14]

Description:

This PR adds the DatadogApiKey detector to the default list of detectors. It gates the new addition behind a feature flag, which is why this PR is based off of #4961 which introduces the plumbing for gating detectors behind feature flags.

Since the flags are disabled by default, the gated detectors need be added to the excludedFromDefaultList of the TestAllDetectorTypesAreInDefaultList test. While doing that, I also separated the gated detectors into their own section (separated by a comment) so that they can easily be identified and removed later.

Testing

Both the unit tests and integration tests pass, no changes required.

Corpora Testing

The Corpora Test in CI was skipped on this PR because no keywords or regex changes were done. Ran the test manually. The detector appears in the list, but a bit below.

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?

---

Note

Medium Risk
Enabling the flag adds another live-verification detector to default scans, which can change findings volume and Datadog API traffic; rollout is mitigated by the feature flag and OSS-only default-on in main.

Overview
Registers the Datadog API key detector in the default engine detector list and wires it into the same feature-flag gating pattern as Pinecone, Cloudinary, GitLab OAuth, and Enigma.

pkg/feature: adds DatadogApiKeyDetectorEnabled. main.go turns it on for OSS builds alongside the other “new detector” flags. defaults.go imports and includes &datadogapikey.Scanner{} in buildDetectorList(), and strips it when the flag is off.

defaults_test.go: keeps DetectorType_DatadogApikey in excludedFromDefaultList while gating applies (so TestAllDetectorTypesAreInDefaultList still passes when the flag is false in tests), and reorganizes exclusions into “mistakenly missed” vs “feature flag gated” sections.

Integration test: unverified Datadog API key expectations now include SecretParts["api_key"].

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[github-actions]

Corpora Test Results

No detector regex or keyword changes in this PR. Bench skipped.