refactor: remove MinerJob defaults to prevent resource exhaustion

[glevco]

Motivation

After running multiple processes that use hathor-core and crash, it's possible that hathor-core will stop working because of leaked resources that are not cleaned up. This happens because of multiprocessing resources that are initialized on module level. The fix is to move them to runtime.

To be clear, this is harmless on most environments, but I caught it while working on integration with Rust.

Acceptance Criteria

• Move default values of MinerJob from module-level to runtime.

---

Below is a traceback when the resources are exhausted:

Traceback (most recent call last):
       File "<string>", line 9, in <module>
       File "./.venv/lib/python3.11/site-packages/hathor_tests/unittest.py", line 16, in <module>
         from hathor.builder import BuildArtifacts, Builder
       File "./.venv/lib/python3.11/site-packages/hathor/builder/__init__.py", line 15, in <module>
         from hathor.builder.builder import BuildArtifacts, Builder, SyncSupportLevel
       File "./.venv/lib/python3.11/site-packages/hathor/builder/builder.py", line 35, in <module>
         from hathor.manager import HathorManager
       File "./.venv/lib/python3.11/site-packages/hathor/manager.py", line 57, in <module>
         from hathor.stratum import StratumFactory
       File "./.venv/lib/python3.11/site-packages/hathor/stratum/__init__.py", line 15, in <module>
         from hathor.stratum.stratum import (
       File "./.venv/lib/python3.11/site-packages/hathor/stratum/stratum.py", line 120, in <module>
         class MinerJob(NamedTuple):
       File "./.venv/lib/python3.11/site-packages/hathor/stratum/stratum.py", line 125, in MinerJob
         data: Any = Array('B', 2048)
                     ^^^^^^^^^^^^^^^^
       File "~/.local/share/uv/python/cpython-3.11.13-macos-aarch64-none/lib/python3.11/multiprocessing/sharedctypes.py",
 line 93, in Array
         lock = ctx.RLock()
                ^^^^^^^^^^^
       File "~/.local/share/uv/python/cpython-3.11.13-macos-aarch64-none/lib/python3.11/multiprocessing/context.py", line
 73, in RLock
         return RLock(ctx=self.get_context())
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "~/.local/share/uv/python/cpython-3.11.13-macos-aarch64-none/lib/python3.11/multiprocessing/synchronize.py",
 line 194, in __init__
         SemLock.__init__(self, RECURSIVE_MUTEX, 1, 1, ctx=ctx)
       File "~/.local/share/uv/python/cpython-3.11.13-macos-aarch64-none/lib/python3.11/multiprocessing/synchronize.py",
 line 57, in __init__
         sl = self._semlock = _multiprocessing.SemLock(
                              ^^^^^^^^^^^^^^^^^^^^^^^^^
     OSError: [Errno 28] No space left on device

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	refactor/miner-job-defaults
Testbed	ubuntu-22.04

Click to view all benchmark results

Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.61 m (-6.23%) Baseline: 1.71 m	1.54 m (95.98%)	2.05 m (78.14%)

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.65%. Comparing base (735f4e0) to head (697e1de).

Files with missing lines	Patch %	Lines
hathor/stratum/stratum.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1604      +/-   ##
==========================================
- Coverage   85.67%   85.65%   -0.02%     
==========================================
  Files         439      439              
  Lines       33618    33618              
  Branches     5282     5282              
==========================================
- Hits        28802    28796       -6     
- Misses       3801     3806       +5     
- Partials     1015     1016       +1     

Flag	Coverage Δ
test-lib	85.65% <83.33%> (-0.02%)	⬇️

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