feat: get plan bulk with cache

[tomerqodo]

Benchmark PR from agentic-review-benchmarks#10

[sentry]

Bug: The function get_plan_bulk_with_cache caches None values for invalid plans, causing subsequent cache reads to fail validation and trigger unnecessary API calls.
_{Severity: MEDIUM}

Suggested Fix

Filter out entries with None values in get_plan_bulk_with_cache before they are added to the cache. Alternatively, modify get_plan_bulk to not include tenants with invalid plans in its returned dictionary.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: api/services/billing_service.py#L365-L367

Potential issue: When a tenant's subscription plan fails validation, `get_plan_bulk`
returns `None` for that tenant. The `get_plan_bulk_with_cache` function then caches this
`None` value as the JSON string `"null"`. On subsequent requests, this cached `"null"`
is read, parsed back to `None`, and fails validation again. This results in a loop of
cache misses and repeated API calls for tenants with invalid data, defeating the purpose
of the cache and increasing API load.

_{Did we get this right? 👍 / 👎 to inform future reviews.}