Add virtualEnvName config to Python hook executor

Background & Motivation
Python hook scripts default to {baseName}_env for virtual environments, but many Python projects use .venv by convention. Issue #7653 explicitly calls out virtualEnvName as a key config property. The Python framework service (framework_service_python.go) also hardcodes the venv name with no config override — hooks should lead in configurability here.

User Story
As a Python developer, I want to specify virtualEnvName in my hook config so that my Python hooks use my preferred virtual environment directory.

Solution Approach

• Read Config["virtualEnvName"] in python_executor.go Prepare() phase via execCtx.Config
• Define a strongly-typed pythonHookConfig struct and unmarshal from execCtx.Config:

  type pythonHookConfig struct {
      VirtualEnvName string `json:"virtualEnvName"` // override venv dir name
  }

• If set, use as the venv name instead of VenvNameForDir() default
• Validate it's a string, non-empty, and doesn't contain path separators (security: prevent path traversal)
• Pass to existing EnsureVirtualEnv() and InstallDependencies() methods

Example azure.yaml

hooks:
  postprovision:
    run: ./hooks/seed.py
    kind: python
    config:
      virtualEnvName: .venv

Acceptance Criteria

• config.virtualEnvName: .venv creates/uses .venv directory for the hook
• config.virtualEnvName: my_env creates/uses my_env directory
• Omitting virtualEnvName preserves current behavior (search .venv/venv, fall back to {baseName}_env)
• Values with path separators (/, \) are rejected with a clear error
• Empty string values are rejected with a clear error
• Config is unmarshalled into a strongly-typed struct (not raw map access)
• Unit tests covering override, fallback, and validation

Default when omitted: Existing behavior — search for .venv/venv in project dir, then fall back to {baseName}_env

Out of Scope

• Python package manager selection (uv, poetry, pdm) — future enhancement
• Custom requirements file name — future enhancement
• Python binary path override — future enhancement

Testing Expectations

• Unit tests: table-driven tests for config unmarshalling, validation, and venv name resolution
• Test path traversal rejection (e.g., ../evil, foo/bar)
• Test empty string rejection

Related Issues

• Depends on Add generic config property bag to HookConfig #7653 / PR Add executor-specific config property bag to hooks with strongly-typed schema #7690 — provides the Config map[string]any on ExecutionContext
• Parent Epic: Multi-Language Hook Support: Python, JavaScript, TypeScript, .NET #7435 — Multi-Language Hook Support