fix: replace and verify native fill values (TKT-996)

[Nikhil (shadowfax92)]

Summary

Fixes TKT-996: filling a prepopulated field could append, and non-editable fields could report success without changing.

act / run fill
  validate editability and obstruction
  establish native focus and select text
  replace by default (clear=false appends)
  insert through the browser editor
  verify the resolved field value or return an error

Both surfaces share the Rust operation. Trusted input events reach page handlers; unreadable/detached fields cannot be mistaken for empty. Existing matching values are idempotent success. Search readiness and login outcomes require application-specific checks.

Evidence

• Before: a real MCP fill of a prefilled input failed exact replacement; a read-only fill incorrectly returned success.
  After: replacement, rejected edits, controlled rollback, empty clearing, native events, shadow/cross-site frame focus and explicit append have browser contracts on both surfaces.
• Root bun run check and bun run test passed; existing lint/Fallow warnings remain.
• Rust workspace tests and Clippy passed. Independent review completed; accepted append finding fixed and verified.
• Pinned Magento fixture with the rebuilt server: xbox is set, trusted input fires, and Search enables after its widget debounce.
• Full real-browser conformance: 135 passed, 0 failed, locally and in Linux CI, using the rebuilt Rust server and isolated browser profiles.

The suite's fixture inventory and an existing stopped-session error assertion were updated to match the actual fixture set and server response.

All checks passed on 0d1c2c935. Linux CI now uses the suite's headless default. Its former Xvfb requirement was for retired hidden-window APIs; the full native-input suite remains enabled.

Merge Danger

Door: two-way; code revert, no schema or data migration.
Blast Radius: MCP fill through act and run. Unsupported controls, rejected/normalized values and wrong focus now produce errors. Fill emits native text-input events; use type/press when keyboard handlers are required.

[github-actions]

✅ Tests passed: 3074/3077

Ran 16 of 16 suites (0 not affected by this change).

Suite	Passed	Failed	Skipped
✅ server-agent	207/207	0	0
✅ server-api	274/274	0	0
✅ server-tools	248/248	0	0
✅ server-browser	10/10	0	0
✅ server-integration	10/10	0	0
✅ server-lib	200/200	0	0
✅ server-root	38/41	0	3
✅ agent	463/463	0	0
✅ claw-app	413/413	0	0
✅ claw-onboard	100/100	0	0
✅ app-onboard	84/84	0	0
✅ build	49/49	0	0
✅ release	66/66	0	0
✅ claw-server-rust	777/777	0	0
✅ claw-server-rust-quality	passed	0	0
✅ claw-mcp	135/135	0	0

passed = ran successfully but emits no JUnit counts (a lint/format gate).

View workflow run

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; no actionable correctness, security, or repository-rule violations remain.

Summary

This PR centralizes fill behavior in the Rust input layer and strengthens its contract:

• Validates editability, visibility, obstruction, focus, and attachment before native insertion.
• Replaces existing text by default while preserving explicit append behavior.
• Reads the resolved field back and reports rejected, normalized, or detached edits as errors.
• Adds real-browser coverage across both MCP surfaces, including contenteditable, shadow DOM, cross-site frames, rejected edits, clearing, and trusted events.
• Runs the browser contract suite using its documented headless default.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[act or run fill] --> B[Resolve ref and frame session]
  B --> C[Validate editable and visible]
  C --> D[Scroll and obstruction check]
  D --> E[Native click and focus]
  E --> F{Replace or append?}
  F -->|Replace| G[Select existing content]
  F -->|Append| H[Collapse selection to end]
  G --> I[Browser-native text insertion]
  H --> I
  I --> J[Read resolved field value]
  J --> K{Matches expected value?}
  K -->|Yes| L[Return success]
  K -->|No| M[Return fill error]

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