Add a validity check for EasyPost API tokens

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

Related to #632650 (closed). Stacked on !259344 (merged), which strips the quotes the rule captures.

Gap

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

Fix

• We generated EasypostClient 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.
• 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.
• The generator wrote the Basic auth headers on one 122-character line:
  • Since rubocop -a split it, it broke generated_client_sync_spec.rb.
  • Therefore, we updated the generator to emit one header per line.

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 endpoint was rejected:
  • A live test key gets 403 with MODE.UNAUTHORIZED there because it requires a production key.

MR acceptance checklist

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