fix(chat): stop an empty localize erasing a conv's topic name

[chrisnojima]

A conv only ever learns its topic name from its max METADATA message (localizer.go:883). A delete-history purges that message's body, so the message survives as valid-but-empty, the type switch matches nothing, and the conv localizes with no name.

Two symptoms, one cause:

• Blank channel name wherever the conv is shown — inbox row, conversation header.
• A team's Emoji tab never loads — FindGeneralConvFromTeamID looks #general up by topic name, and a nameless conv can never match. Same for the bot install modal, which resolves its conversation the same way.

Evidence

Reproduced against a real team whose #general had a delete-history run over it years ago. Instrumenting the localizer to dump each max msg it walks:

msgID: 1  hdrType: METADATA  valid: true  bodyType: NONE   [VALID 1 h:METADATA != b:NONE supBy:1810]

The header still says METADATA; the body unboxes to NONE, because boxer.go:955 treats an empty BodyCiphertext as a deleted body. The message that superseded it is a deletehistory.

So the purge is server-side data: every member of that team localizes the conv nameless, not just a machine with a stale cache. A healthy team's #general localizes to "general" on the same build, which is the control for the harness.

Fix

One hunk, in the localizer: if a conv localizes with no topic name and is marked IsDefaultConv, it is #general whatever its messages say.

That is deliberately the only change. An earlier revision of this PR fixed it at two other sites — MergeLocalMetadata and FindGeneralConvFromTeamID — and measuring that version showed why neither belongs:

• A topic-name query filters on the localized Info.TopicName (inboxsource.go:60), not on the cached copy. Repairing the cache could never make the #general lookup hit; every call still fell through to the IsDefaultConv retry.
• The cached repair only ever fixed the inbox row, leaving the conversation header blank.

Naming the conv where it is localized fixes the lookup, the cached name, and the header at once, so both of those hunks are reverted here.

Scope: this covers a conv marked IsDefaultConv. A non-default channel whose METADATA was purged still localizes nameless, deliberately — its real name is genuinely unrecoverable, so nothing should be invented for it.

Not a regression — this code path is unchanged since v6.6.3, and the topic-name filter since 2020.

Testing

Verified live: services built from master and from this branch, run in turn against the same account, with the conv's inbox cache row deleted between runs so it had to re-localize from the server.

	master	this branch
localized Info.TopicName	""	general
presented channel name	""	general
FindGeneralConvFromTeamID	ERROR: Not found	resolves, channel="general"
team → Emoji tab	renders empty	full emoji list

Also checked the upgrade path, where the cache already holds the empty name an older client wrote. A master build was left to poison the cached copy (Not found, blank tab), then the branch build was swapped in without clearing the cache: the lookup resolved as general and the Emoji tab rendered. So no cache clear or re-sync is needed on upgrade — the cached empty name is not what the lookup filters on, since that query goes remote and localizes.

Unit test added but not run locally — every test in chat and chat/storage signs up a fake user against a dev API server on localhost:3000, so the whole package fails at signup on a dev machine. CI is its first real run.

• TestChatSrvPurgedMetadataDefaultsTopicName — blanks a channel's METADATA BodyCiphertext the way a server-side purge does, re-localizes, and requires the team's default conv to come back as #general while an ordinary channel keeps an empty name rather than getting one invented for it. It then runs the #general lookup that the emoji list and the bot install modal wait on. It pokes IsDefaultConv onto the mock conv, since kbtest never sets that flag on creation; if this goes red, that's the line to look at.

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