Add single header propagation rule

[jeremydmiller]

Summary

• Adds PropagateIncomingHeaderToOutgoing(string headerName) to IPolicies for the common case of forwarding a single named header from incoming to outgoing messages
• Implements PropagateOneHeaderRule — an optimized single-header variant of PropagateHeadersRule that avoids the array allocation
• Expands header propagation documentation with examples for single header, multiple headers, and custom IEnvelopeRule implementations
• Adds cross-reference from the message bus docs to the header propagation page

Example usage:

opts.Policies.PropagateIncomingHeaderToOutgoing("x-on-behalf-of");

Closes #2450

Test plan

• 4 new unit tests for PropagateOneHeaderRule (modify no-op, single header copy, missing header skip, null incoming envelope)
• All 8 header propagation tests pass (4 existing + 4 new)

🤖 Generated with Claude Code