fix(sessions): prevent displayName overwrite on outbound sends

Summary

• Fix session displayName being incorrectly updated to the message recipient when using the message tool to send to a different contact
• Add skipDisplayName option to session metadata functions to prevent this during outbound sends

Fixes #4683

Test plan

• Added unit test for skipDisplayName behavior in metadata.test.ts

Greptile Overview

Greptile Summary

This PR adds a skipDisplayName flag to session metadata derivation so outbound sends don’t overwrite a session’s displayName with the last recipient. The new option is threaded through deriveGroupSessionPatch/deriveSessionMetaPatch, exposed via recordSessionMetaFromInbound, and used by ensureOutboundSessionEntry during outbound message routing. A unit test is added to assert displayName is omitted when the flag is set, and the changelog notes the fix.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• Changes are tightly scoped to gating displayName derivation behind an explicit flag and only applied in the outbound-session entry path; no schema changes and a targeted unit test covers the new behavior. I did not find any functional regressions in the modified call sites.
• No files require special attention

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