fix(slack): restore status across Agent and Assistant API modes

[js37523]

Summary

Fixes the Slack working-status regression reported in #110374 across both Slack API modes.

• Sends processing while working and active when clearing through agents.sessions.setStatus.
• Falls back to the legacy free-text assistant.threads.setStatus only when Slack rejects the Agent Sessions endpoint.
• Remembers that rejection per workspace, avoiding repeated failed calls without degrading other workspaces served by the same adapter.
• Does not retry transient transport failures through the legacy endpoint, preventing duplicate writes.
• Keeps agents.sessions.rename unchanged for thread titles.
• Updates Slack documentation, changelog, package version, and lockfile.

Why this differs from the existing alternatives

Existing PRs for #110374 generally choose either enum-only Agent Sessions status or legacy-only free-text status. This PR combines the compatible parts:

1. Prefer Slack's new Agent Sessions lifecycle contract when it works.
2. Preserve legacy Assistant-view installations by degrading only after a recognized server-side rejection.
3. Scope degradation per workspace and remember it for the process lifetime.

Root cause

Hermes selected agents.sessions.setStatus when slack-sdk 3.44+ exposed the typed method, but continued sending legacy free-text phrases and an empty string. The new endpoint accepts only active | processing | suspended | closed; Slack returned invalid_arguments, which the adapter logged only at DEBUG.

Validation

• RED before implementation: focused routing suite reported 4 failures and 3 passes.
• GREEN after implementation: scripts/run_tests.sh tests/gateway/test_slack.py -q — 251 passed.
• uv lock --check — passed.
• Python compile checks — passed.
• Ruff (when installed) — passed.
• git diff --check — passed.
• Static security scan — no hardcoded secrets, shell injection, eval/exec, pickle, or SQL-formatting findings.
• Independent fail-closed review — passed with no security concerns, logic errors, or suggestions.

Live Slack API verification

Against the affected real workspace/thread:

• agents.sessions.setStatus(status="processing") → success
• agents.sessions.setStatus(status="active") → success
• assistant.threads.setStatus(status="is testing the working indicator...") → success
• assistant.threads.setStatus(status="") → success

Related work

This is a hybrid compatibility alternative to #109914, #110389, #110391, #111820, and #113698. It incorporates the enum contract while retaining a bounded, per-workspace legacy fallback for rejected Agent Sessions installations.

[whyyagswhy]

Independent verification on the PR head (c5eee06): slack suite 250/250 green (1 skipped) on Linux. No findings.