feat: Interrupt running command when LS notifies to do so

Description

Companion MR to gitlab-org/editor-extensions/gitlab-lsp!3123 (merged). This implements the IDE extension side of command cancellation for Duo Workflow commands running in the integrated terminal.

Problem

When a user interrupts a running Duo Workflow command, the LSP sends a $/gitlab/cancelRunningCommand notification, but the IDE extension wasn't handling it. The command continued running in the terminal with no way to stop it or retrieve partial output.

Solution

• Added handler for $/gitlab/cancelRunningCommand notification in TerminalManager
• When cancellation is requested:
  • Sends Ctrl+C (\x03) to the terminal to interrupt the running process
  • Immediately resolves the pending $/gitlab/runCommand request with partial output collected so far

Related Issues

References gitlab-org/editor-extensions/gitlab-lsp#2022 (closed)

How has this been tested?

1. Link this MR and gitlab-org/editor-extensions/gitlab-lsp!3123 (merged)
2. Ask chat to run a long-running command, e.g. "Run npm run test:unit"
3. Approve the command, then click the stop button
4. The terminal process is interrupted with Ctrl+C and partial output is returned to the LSP

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.

What CHANGELOG entry will this MR create?

• feature: New feature - a user-facing change which adds functionality - included in changelog
• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes