Fix composite post-step marker display names

[ericsciple]

Motivation

PR #4243 introduced ##[start-action] / ##[end-action] markers for composite action steps. Two issues remain with the display name used in these markers:

1. Post-step markers show "run" instead of the resolved name. TryUpdateDisplayName skips evaluation during the Post stage, so embedded composite post steps fall back to "run".

2. TryUpdateDisplayName sends unnecessary server updates for embedded steps. Embedded steps share the parent's timeline record, so updating the display name overwrites the parent step's name in the UI.

Fix

Call EvaluateDisplayName directly instead of TryUpdateDisplayName in the composite marker emission path. This resolves the name in-memory without sending a timeline record update. Gated behind existing feature flag actions_runner_emit_composite_markers.

[Copilot]

Pull request overview

This PR fixes display name issues in composite action step markers introduced by PR #4243. The runner now correctly evaluates and displays step names for post-steps in composite actions, and avoids sending unnecessary timeline record updates for embedded steps.

Changes:

• Call EvaluateDisplayName directly instead of TryUpdateDisplayName for embedded composite steps to enable display name evaluation during Post stage
• Add test coverage for Post stage display name evaluation

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Runner.Worker/Handlers/CompositeActionHandler.cs	Replace TryUpdateDisplayName call with EvaluateDisplayName wrapped in try-catch for embedded step display name resolution
src/Test/L0/Worker/Handlers/CompositeActionHandlerL0.cs	Add test verifying Post stage steps evaluate display names correctly (e.g., "Run actions/cache@v4" instead of "run")