refactor: Upgrade to Pydantic v2

[msbrogli]

Motivation

This PR upgrades the codebase from Pydantic v1 to Pydantic v2.

Acceptance Criteria

• Update pydantic dependency from ~1.10.26 to ^2.0 in pyproject.toml
• Migrate @validator decorators to @field_validator with mode='before'/mode='after' instead of pre=True
• Migrate cross-field validators to @model_validator(mode='after') pattern, replacing values dict access with direct model attribute access
• Replace Extra.ignore and other class-level config with model_config = ConfigDict(...)
• Replace .dict() calls with .model_dump()
• Update discriminated unions to use Discriminator() and Tag() from pydantic
• Remove deprecated allow_reuse=True and each_item=True parameters (handled differently in v2)
• Add @classmethod decorator to field validators as required by v2
• Add Hex[bytes] type annotation for automatic hex string parsing

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/pydantic-v2
Testbed	ubuntu-22.04

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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	1.68 m (-1.91%) Baseline: 1.71 m	1.54 m (91.75%)	2.06 m (81.74%)

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[msbrogli]

Can it really be None?

[codecov]

Codecov Report

❌ Patch coverage is 96.34146% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@8cd820c). Learn more about missing BASE report.
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/consensus/poa/poa_signer.py	80.95%	1 Missing and 3 partials ⚠️
hathor/utils/named_tuple.py	75.00%	2 Missing and 1 partial ⚠️
hathor/utils/pydantic.py	90.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1566   +/-   ##
=========================================
  Coverage          ?   85.69%           
=========================================
  Files             ?      441           
  Lines             ?    33793           
  Branches          ?     5286           
=========================================
  Hits              ?    28959           
  Misses            ?     3817           
  Partials          ?     1017           

Flag	Coverage Δ
test-lib	85.69% <96.34%> (?)

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[msbrogli]

Pydantic v2 requires default value to ignore missing parameters. Should these None be included?

The merge-base changed after approval.