fix(test): inject AbortSignal/TextEncoder into miner-extension-live-fetch vm sandbox

[JSONbored]

Summary

• `main`'s CI is currently broken: any PR touching `test/unit/miner-extension-live-fetch.test.ts` (which is most PRs, since it's part of the required test suite) fails on 4-7 tests depending on which code path they exercise. Confirmed live blocking a real, unrelated release-please PR (chore(release): cut engine v3.2.1 #7058, engine v3.2.1).
• Root cause: `node:vm`'s `createContext()` is a fresh, isolated realm with none of the outer process's globals. Two recent, legitimate fixes to `background.js` added real global usage the sandbox never got:
  • `AbortSignal.timeout(...)` bounding the live-fetch call (#4c0b19f4)
  • `new TextEncoder()` measuring the real serialized byte size for the storage-quota guard (fix(miner-extension): guard the live-fetch sync path against the storage quota #7062)
• Every call on the success path threw `" is not defined"`, caught by the function's own `try/catch` and silently misreported as a typed `{ ok: false }` failure result instead of a real one -- which is exactly why this wasn't caught by any assertion on the error message itself, only by the tests expecting the actual success/failure shape.
• Fix: inject both real globals into the sandboxed `context` object, mirroring how `chrome`/`fetch` are already provided.

Test plan

• Reproduced deterministically outside CI (not a flake -- same failure every run, for any candidates payload reaching the success path)
• `npx vitest run miner-extension-live-fetch` -- 15/15 passing after the fix (was 4-7 failing depending on run)
• `npm run test:changed` against `origin/main` -- only this one file affected
• `npm run typecheck` -- clean

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.98%. Comparing base (c8d28b5) to head (05e7af4).
⚠️ Report is 34 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7092      +/-   ##
==========================================
+ Coverage   93.74%   95.98%   +2.23%     
==========================================
  Files         692      609      -83     
  Lines       68706    48144   -20562     
  Branches    18760    15133    -3627     
==========================================
- Hits        64409    46209   -18200     
+ Misses       3302     1118    -2184     
+ Partials      995      817     -178     

Flag	Coverage Δ
rees	?
shard-1	43.34% <ø> (-0.46%)	⬇️
shard-2	36.89% <ø> (+0.04%)	⬆️
shard-3	33.16% <ø> (-0.11%)	⬇️
shard-4	34.39% <ø> (+0.48%)	⬆️
shard-5	31.67% <ø> (-0.57%)	⬇️
shard-6	46.07% <ø> (+0.31%)	⬆️

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