fix: validate volume mount source paths before spawning detached process

[stakeswky]

Summary

Fixes #2485

When running thv run --volume <source>:<target> with a non-existent source path, the command previously appeared to succeed but the workload would silently enter an error state. The actual error was only visible in the proxy log file.

Changes

pkg/runner/config_builder.go — Added pre-flight validation in processVolumeMounts():

• Checks that source paths exist on the host filesystem using os.Stat() (follows symlinks)
• Resolves relative paths to absolute before checking
• Skips validation for Kubernetes operator context (paths are container-relative)
• Skips validation for resource:// URIs
• Returns a clear, actionable error message: volume mount source path does not exist: /path

pkg/runner/config_builder_test.go — Updated existing tests and added coverage:

• Existing volume mount tests now use t.TempDir() for real source paths instead of hardcoded /host paths
• Added test case for non-existent source path (expects error)

Approach

Implemented Option 1 from the issue (validate in processVolumeMounts()) because:

1. It fails at the earliest possible point (during config building)
2. It prevents invalid configs from being created
3. It's context-aware (skips for K8s operator)
4. It aligns with the existing validation pattern in config_builder.go

Before / After

Before:

$ thv run --volume /nonexistent/path:/app playwright
MCP server is running in the background (PID: 12345)
# Silently fails — error only in log file

After:

$ thv run --volume /nonexistent/path:/app playwright
Error: volume mount source path does not exist: /nonexistent/path