fix(vendor): an empty inbound Subject no longer fails a parked step (CL-8590)

[TheGreatAxios]

Summary

• A mail with a present-but-textless Subject: header decoded to subject: "", which createInboundMessage rejects, failing the step and ending the run.
• This is an owner-approved temporary vendored delta, recorded in VENDORED.md with its kill condition.

Changes

• vendor/intx/workflow-host/src/adapters/step-invoker.ts: buildInboundMessageFromMail now omits subject when the trimmed header is empty, matching the existing content guard.
• Exported buildInboundMessageFromMail for direct unit testing.
• VENDORED.md / vendor/intx/workflow-host/VENDORED-FROM: recorded the delta and its kill condition (upstream tolerates an empty Subject, INTR-577).

Testing

• Added a co-located unit test proving a mail with subject: "" produces an inbound message with no subject.
• bun run check passes.

[linear-code]

CL-8590