fix(zig): resume managed embedding backfill after restart

[ajroetker]

Summary

• restore metadata-owned managed embedding providers during startup catch-up
• keep a short-lived startup probe from terminalizing transient provider failures at the persisted retry-budget boundary
• add a focused unit regression and include it in the API table-write test gate

Testing

• zig build -Dedition=full install -fincremental
• focused startup catch-up unit regression: 1 passed
• serial repository regression loop: 10/10 passed
• concurrent repository regression loop: 3 workers x 5 iterations, 15/15 passed
• git diff --check

Context

Follow-up for #347. Reproduced from https://github.com/antflydb/antfly/actions/runs/31531357544/job/93912435099?pr=347.

[ajroetker]

Superseded by commit 96917f1 on #469, where the fix can run against the combined branch and its main-targeting CI.