feat: restructure repository into multi-artifact layout

[ArthurTonial]

Description

Restructures the repository into the multi-artifact layout. All source code and tests are moved from src/ into packages/core/.

Key changes:

• Moved all source modules (auditlog, auditlog_ng, destination, objectstore, secret_resolver, telemetry, ias, aicore) from src/sap_cloud_sdk/core/ and src/sap_cloud_sdk/ into packages/core/sap_cloud_sdk/
• Moved all tests into packages/core/tests/
• Fixed all import paths (sap_cloud_sdk.core.* → sap_cloud_sdk.*) across source modules and test patch strings
• Added packages/core/pyproject.toml as the authoritative build config for the core package
• Moved src/buf/ → packages/core/buf/ (buf-generated validate_pb2 files required by protovalidate)
• Updated Makefile to target the new layout
• Deleted stale src/ tree

Related Issue

Closes #<issue_number>

Type of Change

Please check the relevant option:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

# Build the core package
cd packages/core
uv build

# Run unit tests
uv run pytest -m "not integration"

Expected: wheel and sdist created successfully, 982 tests pass.

Checklist

Before submitting your PR, please review and check the following:

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

If this PR introduces breaking changes, please describe:

• What breaks
• Migration path for users
• Alternative approaches considered

Additional Notes

• The src/ directory is fully removed. The root pyproject.toml still exists but will be replaced with a uv workspace config in a follow-up PR.
• CI/CD workflows still reference src/ paths and will be updated in a follow-up PR once the base branch is merged.