Add per-tab model persistence and selection functionality

[ferenci84]

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Description

Persist agent/mode/model per session (not per tab), and load them when switching tabs. This replaces the previous per‑tab persistence so that a session’s settings travel with it, survive IDE restarts, and are consistent across views.

Original PR (for tabs only): https://github.com/continuedev/continue/pull/7617

Discussion: https://github.com/continuedev/continue/discussions/7986

Key changes:

• Session persistence:
  • Added optional fields to the session payload: mode and chatModelTitle.
  • Persisted to disk in ~/.continue/sessions/<sessionId>.json alongside history/title/workspaceDirectory.
• Save/Load wiring:
  • When saving a session, the GUI includes mode and the selected chat chatModelTitle.
  • When loading a session, the GUI:
    • Restores session.mode into Redux.
    • Restores the selected chat model from session.chatModelTitle (when available).
• Tabs:
  • Removed per‑tab model and mode fields and their reducers (setTabModel, setTabMode).
  • Switching tabs now relies on the session’s persisted data; no more model/mode mirroring to tabs.
• Types and plumbing:
  • Extended Session type (core) with mode? and chatModelTitle?.
  • Updated Core history manager to write/read the new fields when present.

Why

• Keeps a conversation’s configuration together and makes it resilient to reloads.
• Eliminates flicker or divergence from tab-local model/mode.
• Paves the way for persisting additional session-level UI state when needed.

Backward compatibility

• Existing session files load normally (new fields are optional).
• Tabs continue to work; they just no longer store model/mode.

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Checklist

• [X] I've read the contributing guide
• [X] The relevant docs, if any, have been updated or created
• [X] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Persist session-level chat mode and selected chat model. Switching tabs and loading sessions restore the session’s saved mode and model; new sessions start with the current selection.

• New Features
  • Added optional mode and chatModelTitle to Session and persisted them to disk.
  • Loading a session sets Redux mode and updates the selected chat model.
  • Saving a session writes mode and the selected chat model title.

^{Written for commit ea6e871db56dadc53b007c70e2b5fdc52332704a. Summary will update automatically on new commits.}

[ferenci84]

@ferenci84 looks like it's still per-tab as opposed to per session, possible PR is on the wrong branch?

@RomneyDa I just didn't push the latest commit. Corrected now.

[RomneyDa]

Great, will take a look!

[RomneyDa]

@ferenci84 I took forever to follow up on this so I went ahead and touched up. Appreciate the contribution!

[sestinj]

:tada: This PR is included in version 1.5.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[sestinj]

:tada: This PR is included in version 1.31.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[sestinj]

:tada: This PR is included in version 1.35.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[sestinj]

:tada: This PR is included in version 1.6.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: