feat(agent): add workflow revert & redo for AI agent turns

[sshiv012]

What changes were proposed in this PR?

The Texera AI agent edits the user's live workflow canvas, its tools add, modify, and delete operators, and the changes auto-persist. Until now there was no agent-aware way to back out a turn that went wrong; the user had to manually undo a multi-operator change. This PR adds two controls:

• Per-turn "Revert" rewinds the workflow (and the agent-service's HEAD) to the state
  before an agent turn.
• "Redo" undoes the most recent revert; a redo stack supports multiple consecutive
  reverts→redos, and is cleared when a new prompt starts a new branch.

Use case / why: the agent mutates real state, so a bad suggestion (a wrong delete, an unwanted restructure) is expensive to recover from. A cheap, reliable "step back" lowers the cost of letting the agent experiment. A frontend-only visual undo is insufficient, the agent-service keeps its own HEAD, so the next message would silently clobber a cosmetic revert. This does a true rewind that keeps the agent's context consistent.

How it works: the agent-service already models a conversation as a version tree of ReAct steps (id/parentId + beforeWorkflowContent/afterWorkflowContent snapshots + a HEAD pointer).
The only missing primitive was moving HEAD backward.

• Revert(T): HEAD ← parent(T), restore T.beforeWorkflowContent, push old HEAD onto an in-memory redo stack (reverted steps stay in the tree, nothing is deleted).
• Redo: pop the stack, move HEAD forward, restore that step's afterWorkflowContent.
• A new prompt / clear-history clears the redo stack.
• Both commands are rejected while the agent is GENERATING, to avoid racing the loop's own HEAD/workflow mutations.

Protocol additions: WsClientRevertCommand { messageId }, WsClientRedoCommand (client→server); WsServerHeadChangeEvent { headId, workflowContent, canRedo } and a canRedo field on WsServerSnapshotEvent (server→client, so a reconnecting client renders the correct Redo state).

No new persistence, no schema changes, no new services; revert/redo state is the in-memory step tree the agent already maintains.

Scoped as follow-ups: per-individual-step revert, forward/branch navigation UI beyond single-level redo, and persisting revert history across agent-service restarts (currently session-scoped).

Any related issues, documentation, discussions?

Design discussion / RFC: #6039

How was this PR tested?

New automated tests were added and all existing suites pass:

• agent-service (bun): unit tests for revertToTurnStart / redo / canRedo, clear-on-new-prompt, and the WS command handlers including reject-while-generating, unknown-turn, and nothing-to-redo negative cases.
• frontend (vitest): the WsServerHeadChangeEvent handler (HEAD update + canRedo + canvas reload), the revertTurn / redo senders, and the button enable/disable logic.

Full local run of the CI pipeline for both services (rebased on latest main) passes: format / typecheck / bun test (agent-service), and format:ci / build:ci / test:ci / browser-mode tests (frontend).

Manual: agent adds operators → Revert rolls them back → Redo restores them → sending a new message branches and clears Redo.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 4.8)

What changes were proposed in this PR?

Any related issues, documentation, discussions?

How was this PR tested?

Was this PR authored or co-authored using generative AI tooling?

[github-actions]

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[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• Contributors with relevant context: @bobbai00, @Yicong-Huang
  You can notify them by mentioning @bobbai00, @Yicong-Huang in a comment.

[chenlica]

@sshiv012 Thank you. The PR is a "Draft" currently. Please mention @bobbai00 after it's ready to review.