fix: handle nullable message metadata

[steipete-oai]

Summary

• normalize nullable message metadata to empty strings across search, message listing, and thread-context reads
• cover nullable user, thread, reply, and subtype fields with a regression test
• document the fix for the next patch release

Verification

• GOWORK=off go test ./...
• structured Codex autoreview: clean, no accepted/actionable findings

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix: handle nullable message metadata This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.

[steipete]

Reviewed the null-normalization paths and regression coverage. Full Go tests, CI, CodeQL, secret scanning, and structured autoreview are clean.