fix(integrations): use Datadog MCP header names

[neubig]

I have confirmed that this works in agent canvas.

• A human has tested these changes.

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Why

The Datadog integration catalog used the REST API-style DD-API-KEY and
DD-APPLICATION-KEY request headers for the hosted MCP server. Datadog's MCP
documentation requires DD_API_KEY and DD_APPLICATION_KEY, so clients that
send the catalog fields verbatim receive a 403 response.

Summary

• Use Datadog's documented hosted MCP API-key header names.
• Point the integration at the current Datadog MCP setup documentation.
• Add a regression test for the Datadog MCP header contract.

Issue Number

N/A

How to Test

npm run build:integrations
python scripts/sync_extensions.py --check
uv run pytest -q

The full test suite passes: 461 tests.

Video/Screenshots

N/A - catalog metadata only.

Notes

Datadog's ordinary REST API continues to use the hyphenated headers. This
change is limited to the hosted MCP integration metadata.

Tracking

Related to #363.

Tracking issue

Closes #411.

Live evidence

Validated the hosted Datadog MCP production endpoint on 2026-07-14 UTC using the corrected underscore header names. Credentials were sent only as redacted request headers.

• Endpoint: https://mcp.us5.datadoghq.com/v1/mcp
• JSON-RPC initialize returned HTTP 200, protocol 2025-06-18, server datadog-mcp version 1.0.0, and tools capability.
• A session-scoped tools/list returned HTTP 200 with 3 tools: get_change_stories, list_datadog_skills, and load_datadog_skill.
• The attached Agent Canvas screenshot and checked human-testing attestation remain the UI-path evidence.
• Repository validation: npm run build:integrations, python scripts/sync_extensions.py --check, and uv run pytest -q (461 tests).