Consolidate poe tasks

[AdrienVannson]

Description

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation / tooling

Checklist

• New behaviour is covered by tests
• Documentation updated if needed

[gaoflow]

I checked the failing Check running without dependencies job. The immediate failure is not in the tests themselves; after the task consolidation, the root-level poe test-lib task runs pytest from the root uv run poe ... environment, and that environment only has the root dev group (poethepoet), not the betterproto2 package test group:

Poe => pytest
error: Failed to spawn: `pytest`
  Caused by: No such file or directory (os error 2)

Before this PR, CI ran uv run poe test with working-directory: ./betterproto2, so uv resolved the betterproto2 project/default test dependencies before launching pytest. A minimal way to preserve the centralized root Poe tasks while restoring that behavior is to let each test task invoke package-local uv run from its configured cwd:

 [tool.poe.tasks.test-lib]
-cmd = "pytest"
+cmd = "uv run pytest"
 cwd = "betterproto2"

 [tool.poe.tasks.test-lib-cov]
-cmd = "pytest --cov=betterproto2 --cov-report=term --cov-report=html tests/"
+cmd = "uv run pytest --cov=betterproto2 --cov-report=term --cov-report=html tests/"
 cwd = "betterproto2"

 [tool.poe.tasks.test-plugin]
-cmd = "pytest"
+cmd = "uv run pytest"
 cwd = "betterproto2_compiler"

I verified with poe --dry-run test-lib / test-plugin that the root tasks still resolve after this change. I could not push a commit here because my repo permission is read-only.