feat(cells) Implement most of the org-cell-mappings endpoint

[markstory]

A core feature of synapse is to route requests between cells. In order
to do that routing, synapse will need to periodically fetch org mapping
data allowing it to map slugs + ids to the regions.

Refs INFRENG-171

[linear]

INFRENG-171 Create /org-cell-mappings API endpoint

General approach: avoid exposing the raw exception message to the client. Instead, convert BadPaginationError into a ParseError (or similar 4xx error) with a generic, client-safe message. Optionally log the original exception server-side, but do not include it in the HTTP response.

Best fix here without changing existing functionality: keep raising ParseError, but change detail=str(e) to a constant generic message like "Invalid pagination parameters.". This preserves the HTTP error type and semantics (client gave bad input) while ensuring no internal details are leaked. If desired, server-side logging of e could be added, but since no logging utilities are shown in this snippet, we will only adjust the detail string.

Concretely, in src/sentry/synapse/endpoints/org_cell_mappings.py, within the get method of OrgCellMappingsEndpoint, update the except BadPaginationError block. Replace:

except BadPaginationError as e:
    raise ParseError(detail=str(e))

with:

except BadPaginationError:
    raise ParseError(detail="Invalid pagination parameters.")

No new imports or helper methods are required.

[cursor]

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