feat(python): add core 3.14/3.15 gate scenarios (PROF-15511)

[vlad-scherbich]

Prev: #165 | Next: #169

Motivation

First scenario batch for the 3.14/3.15 migration gate. Covers lock, exception, and async-generator profiler families — workloads that exercise event labels and wall-time collection across Python minor versions.

Summary

Adds 6 scenario directories (3 families × 2 Python versions):

Family	3.14	3.15	Key assertions
lock	python_lock_3.14	python_lock_3.15	lock-acquire, lock-release, lock name
exceptions	python_exceptions_3.14	python_exceptions_3.15	exception type, wall-time
async-gen	python_async_gen_3.14	python_async_gen_3.15	async-generator frames, wall-time

• Updates gate regexp in python_downstream_gate/README.md: python_(exceptions|async_gen|lock)_3\.(14|15)
• Lock/exception label hardening included here (supersedes separate label-only PRs)
• python_*_3.15 dirs are wheel-only — run via dd-trace-py downstream with DDTRACE_INSTALL_URL

Gate size after merge: 6 scenarios.

Test plan

• go test -run TestSchemaValidation
• ruff check on scenario sources
• Downstream gate with DDTRACE_INSTALL_URL