chore: move this project to V4

[kessplas]

Issue #, if available:

Description of changes: Since this release supports Key Commitment, it should be V4 to be relatively more consistent with other S3EC versions. However, semantic release will want to do a version bump, so we will make it 3.0.0 for now, so it will be 4.0.0 upon release.

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[Copilot]

Pull request overview

This PR updates the repo and its cross-language test framework to treat the Python implementation as “V4” (key commitment capable), adds a new FastAPI-based Python V4 test server, and updates integration-test wiring/docs to target that V4 server.

Changes:

• Add test-server/python-v4-server (FastAPI server + build/run scaffolding) and update test-server docs accordingly.
• Update Java integration tests to reference Python-V4 and to scope key-commitment failure tests to Python V4.
• Bump the library version to 3.0.0 (per PR description’s semantic-release workflow) and update CI caching to point at the new Python V4 server pyproject.

Reviewed changes

Copilot reviewed 7 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test-server/README.md	Updates documented commands to reference Python V4 server (but currently documents non-existent Make targets).
test-server/python-v4-server/src/main.py	Adds FastAPI implementation of the Python V4 test server (PUT/GET object + client creation).
test-server/python-v4-server/src/init.py	Adds package marker for server source.
test-server/python-v4-server/tests/init.py	Adds package marker for server tests.
test-server/python-v4-server/README.md	Documents setup and usage for the Python V4 server.
test-server/python-v4-server/pyproject.toml	Defines the Python V4 server project metadata/dependencies.
test-server/python-v4-server/poetry.lock	Adds a lockfile for the Python V4 server dependencies.
test-server/python-v4-server/Makefile	Adds build/start/stop targets for the Python V4 server.
test-server/python-v4-server/.gitignore	Ignores venv/build/test artifacts for the Python V4 server.
test-server/python-v4-server/.duvet/config.toml	Adds duvet configuration for spec coverage reporting.
test-server/python-v4-server/.duvet/.gitignore	Ignores generated duvet reports/inputs.
test-server/java-tests/src/it/java/software/amazon/encryption/s3/TestUtils.java	Renames Python target constants to Python-V4 and updates server map/feature sets accordingly.
test-server/java-tests/src/it/java/software/amazon/encryption/s3/KeyCommitmentPolicyEncryptFailureTests.java	Switches MethodSource from Python V3 to Python V4.
src/s3_encryption/init.py	Updates top-level package docstring to “v4”.
pyproject.toml	Bumps package version to 3.0.0.
.github/workflows/python-integ.yml	Updates uv cache key hashing to use the Python V4 server pyproject path.

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