Add a validity check for EasyPost test API tokens

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This merge request adds a validity check for EasyPost test API tokens.

Resolves #632650 (closed). Stacked on !260411 (merged), which adds the EasyPost API token check this one mirrors.

Gap

• GitLab detects EasyPost test API token (rule), but validity checks did not verify it.

Fix

• We generated EasypostTestClient and its spec from a vendor definition using the generator.
• The check calls GET /v2/addresses with the key as the Basic auth username, and is only enabled for GSS findings.
• It shares the endpoint, status mapping and partner_easypost_api rate limit with the EasyPost API token check.
• Every mapping decision was measured against real production and test keys:
  • Live production keys return 200.
  • Disabled, deleted and unknown keys return 403 with APIKEY.INACTIVE.

Notes

• 401 stays unmapped and reads as unknown:
  • EasyPost returned it only for requests without a key.
  • EasyPost errors guide also uses UNAUTHORIZED for credentials that "do not permit access".
• /v2/users was rejected:
  • A live test key gets 403 with MODE.UNAUTHORIZED there, so this check would read every live test key as inactive.

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.