Add job execution view model

[rentziass]

Part 1 of 2 of adding a job execution view for the DAP debugger. Followed by #4471.

This PR introduces a typed JobExecutionView model so we can define and review the rendered job view separately from the debugger plumbing that will serve it.

The reason for splitting it this way is that the view has its own behavior and invariants:

• it needs to render a stable synthetic source for the job
• it needs to preserve source lines so the debugger can point at the right step later
• it needs to model pre/main/post sections, predicted post steps, and the synthetic Complete job row consistently

Putting that behavior behind a dedicated type keeps the follow-up DAP integration PR much smaller and lets us validate the output shape directly in focused tests.

Example rendered job view:

pre:
  - step: "Setup job"
  - step: "Pre cache"

main:
  - step: "Checkout"
  - step: "Build"

post:
  - step: "Post cache"
  - step: "Complete job"

[Copilot]

Pull request overview

Introduces an internal JobExecutionView model under Runner.Worker.Dap to render a stable synthetic “job execution” YAML-like source (with deterministic line numbers) that the DAP debugger can later serve and map stack frames/pauses back onto. This is the rendering/model half of the planned 2-part change, with focused L0 tests validating the output shape and line mapping invariants.

Changes:

• Add JobExecutionView renderer that splits steps into pre/main/post, supports predicted post steps, and tracks per-step line numbers plus a synthetic “Complete job” line.
• Add L0 tests covering section rendering, predicted post-step claiming without line shifts, and “Complete job” name collision behavior.
• Normalize InternalsVisibleTo.cs file header/encoding (no functional behavior change).

Show a summary per file

File	Description
src/Runner.Worker/Dap/JobExecutionView.cs	New typed model that renders the synthetic execution view and preserves step→line mappings for later DAP source/stack mapping.
src/Test/L0/Worker/JobExecutionViewL0.cs	New L0 tests asserting rendered content and stable line assignments, including predicted post-step claiming behavior.
src/Runner.Worker/InternalsVisibleTo.cs	Minor file header/encoding change; no behavioral impact.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 3