Add Python startup environment hook

[mikemegally-ms]

Add an optional host-supplied script as an ordering point after Python environment, package, and virtual environment setup and before existing pre-run and application commands.

Preserve generated startup behavior when the option is omitted or empty, and propagate sourced-script failures so later commands do not run.

Add unit and Linux runtime coverage for ordering, quoting, legacy compatibility, environment visibility, and failure handling, and document the public command contract.

• The purpose of this PR is explained in this message or in an issue. If an issue please include a reference as #<issue_number>: the PR body explains the optional Python startup-environment hook, its ordering, compatibility, and failure behavior. No issue reference is required.
• Tests are included and/or updated for code changes: added:
• startupenvironment_test.go
• startupenvironment_linux_test.go
  Targeted Go tests passed: GOTOOLCHAIN=local go test -count=1 ./...
• Proper license headers are included in each file: all four changed/new Go files contain the standard Microsoft copyright and MIT license header. doc/runtimes/python.md follows the repository’s documentation convention and does not require a source-file header.