feat: add local clients for destination module

[NicoleMGomes]

Description

Adds local development mode for the destination module. When a mocks/<resource>.json file is present at the repo root, the factory functions (create_client(), create_fragment_client(), create_certificate_client()) automatically return file-backed local clients that implement the same interface as the cloud clients — no credentials or network access required. Removing the mock file reverts to cloud mode with no code changes.

A shared abstract base class (LocalDevClientBase) handles JSON file I/O, thread-safe writes, and access-strategy resolution (SUBSCRIBER_ONLY, PROVIDER_ONLY, SUBSCRIBER_FIRST, PROVIDER_FIRST) for all three local clients. A WARNING-level log is emitted whenever a local client is returned so developers are aware they are not hitting the real service. mocks/ is added to .gitignore to prevent accidental commits of mock files that may contain sensitive data.

Related Issue

Type of Change

• 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

1. Clone the repo and install dependencies: uv sync --dev
2. Create the mock files to activate local mode:

   mkdir -p mocks
   echo '{"subaccount":[],"instance":[]}' > mocks/destination.json
   echo '{"subaccount":[],"instance":[]}' > mocks/fragments.json
   echo '{"subaccount":[],"instance":[]}' > mocks/certificates.json

3. Run the smoke scripts:

   uv run python main_destination.py
   uv run python main_certificate.py
   uv run python main_fragment.py

4. Expected result: each script prints Client type: LocalDev<Resource>Client and completes the full create → get → update → list → delete cycle without errors or network calls.
5. Remove the mock files and confirm create_client() falls back to cloud mode (requires CLOUD_SDK_CFG_* env vars or /etc/secrets/appfnd mount).
6. Run unit tests: uv run pytest tests/destination/unit/ -v

Checklist

• 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

None. Local mode is additive — the factory functions behave exactly as before when no mock files are present.

Additional Notes

• Mock file format: {"subaccount": [...], "instance": [...]}. Subaccount entries without a "tenant" field are provider entries; entries with "tenant" are subscriber-specific.
• The detection is based on file presence at <repo_root>/mocks/<resource>.json. This makes local mode opt-in at the filesystem level with no environment variable or code change required to toggle it.
• mocks/ is gitignored to avoid accidental commits — mock files may contain local URLs, certificate content, or other sensitive data.
• New unit tests cover all four access strategies, both scopes (instance / subaccount), conflict (409) and not-found (404) error cases, and the factory auto-detection logic in test_init.py.

[LucasAlvesSoares]