fix: use empty defaults for OAuth-mode API key headers

[nicoyoung]

What does this PR do?

The MCP server config declares the DD_API_KEY and DD_APPLICATION_KEY headers as ${DD_API_KEY} / ${DD_APPLICATION_KEY} with no default. The plugin authenticates via OAuth by default, so these are normally unset, and Claude Code's static MCP-config validator treats a ${VAR} reference without a default as a required variable. On a default (OAuth) install it therefore reports the server as invalid:

MCP server mcp invalid: Missing environment variables: DD_API_KEY, DD_APPLICATION_KEY

This surfaces in /doctor for every OAuth user even though the server connects and works. This PR gives the two headers empty defaults (${DD_API_KEY:-} / ${DD_APPLICATION_KEY:-}), matching how ${DD_MCP_TOOLSETS:-} is already handled in the same file. Behavior is unchanged: when the variables are set (advanced API-key usage) their values are used; when unset (OAuth) they resolve to empty strings instead of being flagged as missing.

Checklist

• Tests added or updated where applicable (n/a, config-only change)
• Documentation updated where applicable (n/a)
• No secrets, internal URLs, or other internal references included

Closes #18

[nicoyoung]

Friendly ping @datadog-labs/mcp-server / @m-paternostro — this fixes the not-setup/MCP-domain issue described in #14 for OAuth-mode users. Happy to address any feedback if a maintainer can take a look. Thanks!