feat: keep Copilot BYOK agent turns alive in server Agent Host

[khanhkit]

Fixes #8046

Motivation

code-server is a remote IDE, so long-running agent work should be owned by a server-side lifecycle rather than by a browser tab. For direct-capable Copilot Chat BYOK providers, this change lets the remote Agent Host own model execution so an active turn is not tied to renderer/tab lifetime.

Design

• Direct-capable BYOK providers are synchronized to Agent Host as bounded, ephemeral descriptors and registered directly with the server-side Copilot SDK.
• Provider configurations that cannot be represented faithfully stay on the existing renderer bridge rather than being silently normalized.
• Credential-bearing syncEphemeralByokModels data stays process-memory-only and is centrally redacted before AHP JSONL serialization.
• Model capability metadata (vision, tool calling, reasoning effort) is preserved.
• Agent Host Changes-pill defaults follow isolation when there is no provider override: folder -> Session Changes, worktree -> Branch Changes.
• Native Copilot authentication semantics are not bypassed; this change is for the direct BYOK path.

Files changed

• patches/copilot-background-byok.diff
• patches/copilot-agenthost-session-changes.diff
• patches/series

Current VS Code 1.141 verification

Current PR head: 73f71d62f4b26c04dd50b595a3837231a1b5b6aa

Pinned VS Code gitlink: 2a59476c9bfcb90b3ddc372c36762471b7dfad1c (VS Code 1.141)

Executed on the 1.141 port:

• full code-server patch series clean apply: 31/31;
• regenerated feature patches reproduce 22/22 target files byte-for-byte from the validated scratch tree;
• copilotSessionLauncher.test: 109/109 pass;
• byokLmBridgeRegistry.test: 11/11 pass;
• protocolServerHandler.test: 175/175 pass;
• ahpJsonlLogger.test: 14/14 pass, including the credential-redaction sentinel;
• browser AgentHostByokLmHandler: 22/22 pass;
• browser AgentHostSessionInputPills: 22/22 pass, including folder -> Session Changes.

Exact-head isolated ARM64 release verification is now GREEN for 73f71d62f4b26c04dd50b595a3837231a1b5b6aa: full build:vscode completed (including core-ci desktop/server/server-web bundles), code-server build completed, release materialized successfully, and a fresh out-of-container checksum verification passed 5897/5897 files. The built runtime reports 73f71d62f4b26c04dd50b595a3837231a1b5b6aa with Code 1.141.0.

GitHub Actions status

Upstream Build run 37796020383 was created for exact head 73f71d62f4b26c04dd50b595a3837231a1b5b6aa, but GitHub currently reports action_required with zero jobs created. The workflow is waiting for the repository's fork-workflow approval; no CI test has executed or failed yet.

Historical note

The earlier VS Code 1.140 candidate 95c0eb85d4265925751949f729851901e8e9db0d passed its isolated build, fork CI, and live background/detach acceptance. Those results are historical evidence only and are not presented as exact-head verification for this 1.141 port.

[code-asher]

Thank you for the PR! The reasoning makes sense, but this looks like a significant change. Could we propose this change upstream directly rather than try maintaining it as a patch here? I think the VS Code maintainers would find it valuable as well, since presumably Codespaces has the same problems, or anyone running regular VS Code in the web.

[khanhkit]

Exact-head VS Code 1.141 verification update:

• candidate: 73f71d62f4b26c04dd50b595a3837231a1b5b6aa
• full build:vscode: PASS (core-ci, desktop/server/server-web bundles)
• code-server build + release: PASS
• fresh release checksum verification: 5897/5897
• runtime identity: exact candidate with Code 1.141.0

Upstream Actions run 37796020383 is still action_required with zero jobs, so it is awaiting repository approval for the fork workflow; no upstream CI test has failed.