## What changed
Move shell command construction, bounded process execution, and JSON header parsing from `rmcp-client` into `codex-http-headers-helper`. Preserve the 10-second timeout, 64 KiB output limit, process-tree cleanup, and rejection of invalid or duplicate headers. Keep MCP environment setup and reserved-header validation in `rmcp-client`.
Mark helper-provided header values as sensitive so debug formatting redacts credentials.
## Testing
Add parser tests for credential-safe errors and debug redaction, and adapt the MCP reserved-header test to exercise helper execution.
GitOrigin-RevId: e32d96261ff3648722014971bbf3803d37176ab4
