Add isolated workspace resolver for flow run setup

[desertaxle]

refs #19321

this PR adds an internal workspace resolver primitive for safely preparing deployment workspaces before later uv launcher integration.

Details

• introduces private prefect.runner._workspace_resolver subprocess entrypoint that prepares storage and pull-step workspaces and returns structured metadata as JSON
• preserves parent cwd by running resolution in a child process, keeping stdout reserved for JSON and redirecting pull-step output to stderr
• tracks pull-step cwd outputs, local path project roots, environment, and sys.path metadata for later executor wiring
• adds a narrow optional run_steps completion callback with best-effort cwd probing only when the callback is supplied
• covers local storage, git clone, remote and block storage, custom cwd behavior, noisy stdout, process state capture, structured failures, and deleted cwd recovery

Reviewer example

For a deployment whose pull steps clone a repo and then enter a nested src directory:

pull:
- prefect.deployments.steps.git_clone:
    id: clone
    repository: https://github.com/acme/data-jobs.git
    clone_directory_name: repo
- prefect.deployments.steps.set_working_directory:
    directory: "{{ clone.directory }}/service/src"

deployments:
- name: daily-report
  entrypoint: flows/report.py:daily_report

the resolver can be invoked before the eventual flow subprocess:

python -m prefect.runner._workspace_resolver "$FLOW_RUN_ID" "/tmp/prefect-workspaces/$FLOW_RUN_ID"

It prepares the pull-step workspace and returns metadata like:

{
  "status": "success",
  "workspace": {
    "workspace_root": "/tmp/prefect-workspaces/abc",
    "working_directory": "/tmp/prefect-workspaces/abc/repo/service/src",
    "project_root": "/tmp/prefect-workspaces/abc/repo/service",
    "runtime_entrypoint": "flows/report.py:daily_report"
  }
}

A uv-aware executor can then run from working_directory while using project_root for project dependency resolution:

cd /tmp/prefect-workspaces/abc/repo/service/src

PREFECT__FLOW_RUN_ID="$FLOW_RUN_ID" \
PREFECT__FLOW_ENTRYPOINT="flows/report.py:daily_report" \
uv run --project /tmp/prefect-workspaces/abc/repo/service \
  python -m prefect.engine

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

---

_{Comparing feat/first-class-uv-support (b7a66ac) with main (efb35bf)}

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