Resolves: #324648
Mirror main: #324672

Regressed by: #323914

• Add explicitSessionType?: string to IChatEditorOptions so callers can preserve an explicit session picker choice when opening a new chat editor.
• Set explicitSessionType: localChatSessionType in openChatSession only when the user picks Local for an editor session.
• Make ChatEditorInput.resolve() honor explicit Local before applying last-used-agent/default-provider resolution, so switching from Copilot AH to Local starts a local session instead of resolving back to Copilot.
• Make shouldReplaceEmptyLocalSession() skip replacing an empty session that was explicitly opened as Local, so it is not swapped back to the default provider on a later resolve.
• Add unit tests covering both paths: starting a fresh explicit Local session, and preserving an already-resolved empty Local session.

Regression history:

• chat: restore last-used agent and model for new chat editors #323011 introduced remembering the last-used non-local agent for new chat editors (34a9b07).
• Remove last-used session type and agent logic #323484 removed that last-used session type / agent logic (b802a62), and the removal was also cherry-picked to release/1.127 as Remove last-used session type and agent logic #323555.
• Automations: Management UI (create/edit dialog, list widget, ChatInputPart integration) #323914 reintroduced the last-used session type behavior removed by Remove last-used session type and agent logic #323484. Once the session is released, a new chat editor opened from the picker can inherit the last selected non-local agent, such as Copilot AH, instead of staying on the intended local agent.
• Local editor opens use a generic vscode-chat-editor resource, so the editor input could not distinguish "the user explicitly picked Local" from "open a normal new chat editor." This PR preserves that explicit Local intent.

Evidence:

• Automations: Management UI (create/edit dialog, list widget, ChatInputPart integration) #323914 reintroduced storing the last-used non-local session type in ChatWidgetService: recordLastUsedSessionType, called from setLastFocusedWidget and the view-model change listener. Note recordLastUsedSessionType intentionally never stores local, so an explicit Local picker choice needs a separate signal to bypass last-used non-local resolution.
• Automations: Management UI (create/edit dialog, list widget, ChatInputPart integration) #323914 changed ChatEditorInput.resolve() back to reading ChatLastUsedEditorSessionTypeStorageKey and calling getNewChatEditorSessionResource(...): chatEditorInput.ts#L236-L237.
• getNewChatEditorSessionType(...) prefers that last-used non-local session type when chat.editor.defaultProvider is not explicitly configured: constants.ts#L319-L324.
• This PR fixes the missing intent signal by setting explicitSessionType from openChatSession and checking it before the last-used/default-provider path in ChatEditorInput.resolve().

Inspirations from:

• The explicitSessionType branching in resolve() follows the existing options.target handling in the same method: ChatEditorInput.resolve().
• The shouldReplaceEmptyLocalSession() guard extends the existing predicate rather than adding a new code path: shouldReplaceEmptyLocalSession.

Validation:

• git diff --check clean.
• npm run typecheck-client fails on unrelated src/vs/workbench/api/node/proxyResolver.ts(115,52) / resolveProxyByURL, which is outside this PR's touched files.
• Added two unit tests in chatEditorInput.test.ts: explicit Local resolves to a fresh local session without consulting the default/last-used path, and an empty explicitly-Local session is preserved instead of replaced.

Manual test:

1. Open a Copilot (AH) chat editor, then use the picker to switch to Local -> stays Local, no flicker back to Copilot.
2. Repeat with chat.editor.defaultProvider set to a non-local provider -> explicit Local still sticks, including after a window reload with the empty session open.
3. Open a plain new chat editor without an explicit picker choice -> existing default/last-used behavior is unchanged.

/cc @benvillalobos