Add app log stream affinity for scalable apps

[jerrylian-db]

Changes

• Warn when databricks apps logs is used with an app configured for more than one instance.
• Send a per-command __Host-databricks-app-router cookie on log websocket handshakes for scalable apps.

Why

databricks apps logs --follow reconnects the websocket on transient failures. A stable routing cookie keeps reconnects best-effort affined to the same app instance, matching scalable Apps session affinity guidance.

Tests

• go test ./cmd/apps ./libs/apps/logstream
• ./task checks
• ./task lint-q (Go lint passed; Python ruff fetch failed because PyPI DNS resolution failed)
• ./task test (unit test phases passed; acceptance phase blocked because local jq is 1.6 and the suite requires jq >= 1.7)

This PR was written by Codex.

[github-actions]

Approval status: pending

/cmd/apps/ - approved by @pkosiec

Files: cmd/apps/logs.go, cmd/apps/logs_test.go

/libs/apps/ - approved by @pkosiec

Files: libs/apps/logstream/streamer.go, libs/apps/logstream/streamer_test.go

General files (require maintainer)

Files: .nextchanges/cli/apps-logs-scalable-affinity.md
Based on git history:

• @rugpanov -- recent work in .nextchanges/cli/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}

[eng-dev-ecosystem-bot]

Integration test report

Commit: 63565f6

Run: 35370433217

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	15	4:25
✅​	aws windows	278	13	4:09
✅​	azure linux	275	15	4:10
✅​	azure windows	277	13	3:37
✅​	gcp linux	276	15	4:18
✅​	gcp windows	278	13	5:23

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
5:21	gcp windows	TestAccept
4:06	aws windows	TestAccept
3:36	azure windows	TestAccept

[pkosiec]

Overal LGTM, nice work 👍

Are there any plans to allow users to connect to a specific replica with a flag? That would be awesome.

[github-actions]

An authorized user can trigger integration tests manually by following the instructions below:

Trigger:
go/deco-tests-run/cli

Inputs:

• PR number: 6742
• Commit SHA: 63565f6e2c109d3bd4345932db1619d540ea8485

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