Add configurable max file size limit for LOOKUP content retrieval

[mchades]

Context

LocalFSBackend._execute_lookup() calls read_bytes() with no upper bound on file size, which can OOM the server on very large binary files.

Proposed Approach

Add a max_file_size config field to BlobStorageBackendConfig (e.g., default 100 MB) and check file size before reading content.

References

• PR [#46] feat(backends): add BLOB_STORAGE backend with local filesystem provider #47 review comment by @jerryshao