Complete fail-safe miner monitor state loading

[nwinkelman2]

Summary

• close the post-merge review gap identified on fix(miner-notifications): atomic state writes, robust load, SMTP leak fix (H-04, H-05, H-06) #16791
• reject a valid JSON document when its top-level value is not an object
• reject a non-object miners collection before calling .items()
• preserve the existing per-entry skip behavior for malformed miner records

Why

The original hardening caught JSON syntax errors and malformed individual rows, but structurally valid JSON such as [] or {"miners": []} still raised AttributeError outside the per-entry guard and aborted monitor startup.

Proof

• targeted wrong-shape + save/load tests: 4/4 passed
• complete miner monitor suite: 24/24 passed
• Python compile and diff-integrity checks clean

Follow-up to the owner change request on #16791: #16791 (review)