feat(cache): add core cache module

[cassiofariasmachado]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Adds sap_cloud_sdk.cache — a domain-agnostic, pluggable cache layer to be shared across all SDK modules. The module is infrastructure-only: it has no BTP service binding and no create_client() factory.

Key design decisions follow the cache core strategy spec:

• CacheBackend ABC — four-method interface (get, set, delete, clear); fully backend-agnostic (no tenant or TTL awareness).
• InMemoryLRUBackend — default in-process backend backed by cachetools.TTLCache; thread-safe; per-entry TTL via monotonic clock; LRU eviction at max_size.
• CacheConfig — dataclass covering enabled, isolation_strategy, default_ttl_seconds, expiry_buffer_seconds, max_size, backend, and on_evict callback.
• configure_cache() / get_cache_config() — global registry set once at startup; per-client override supported via CacheConfig constructor argument.
• IsolationStrategy — TENANT / TENANT_USER enum; auto-selected from context (user ID present → TENANT_USER); SHA-256-based key derivation prevents cross-tenant and cross-user cache hits.
• Cache façade (SDK-internal, not exported) — glues config, isolation, and backend; applies expiry_buffer_seconds; isolates backend exceptions.

Not in scope for this draft: migrating existing per-module token caches (adms, agentgateway), Redis backend implementation, destination caching integration.

Related Issue

Type of Change

• New feature (non-breaking change that adds functionality)
• Dependency update

How to Test

1. Install dependencies: uv sync --all-extras --group dev
2. Run the cache unit tests: uv run pytest tests/cache/ -v
3. Run the full lint and type-check suite:

   uv run ruff check src/sap_cloud_sdk/cache/
   uv run ruff format --check src/sap_cloud_sdk/cache/
   uv run ty check src/sap_cloud_sdk/cache/
   

4. Expected result: 53 tests pass, all checks clean.

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 — this is a purely additive new module.

Additional Notes

• cachetools~=5.5.2 added to [project.dependencies] (MIT licence, already a transitive dependency of google-auth in the ecosystem).
• Module.CACHE = "cache" added to core/telemetry/module.py (alphabetical order). No operation constants needed — the cache is infrastructure, not a traced service call.
• The Cache façade is intentionally not exported from sap_cloud_sdk.cache; SDK modules import it directly via from sap_cloud_sdk.cache._cache import Cache.

[NicoleMGomes]

I like it to have customization.

Do we know if like redis cache could be inplemented?

Can we also offer a more global configuration at least for Cache class to be used?

[NicoleMGomes]

Is this mandatory? If yes, we would need a provider level.

[NicoleMGomes]

Can we implement it in one module to exemplify usage?