feat: Add Ethereum-style timestamp validation and PoW hash check

[SIDDHANTCOOKIE]

Addressed Issues:

This PR introduces critical security and consensus fixes to the Blockchain class to prevent difficulty manipulation and secure the Proof-of-Work validation loop.

The Vulnerability Fixed: Previously, validate_block_link_and_hash verified that a block's hash mathematically matched its contents, but it never checked if the hash actually satisfied the target difficulty. A malicious actor could broadcast a block with nonce=0 and as long as they set the difficulty integer to the required target, the network would accept it instantly without any actual PoW being performed.

Screenshots/Recordings:

TODO: If applicable, add screenshots or recordings that demonstrate the interface before and after the changes.

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Summary by CodeRabbit

• Bug Fixes

  • Added stricter block validation to reject links with invalid proof-of-work, non-increasing timestamps, or timestamps too far in the future.
  • Improved rejection handling so invalid blocks are classified more accurately.

• Tests

  • Updated persistence tests to use deterministic block timestamps for more reliable results.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 30b952fb-e731-425c-a981-200885e1447f

📥 Commits

Reviewing files that changed from the base of the PR and between 9419fdb and 1335026.

📒 Files selected for processing (2)

• minichain/chain.py
• tests/test_persistence.py

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Walkthrough

Block validation in chain.py now enforces Proof-of-Work difficulty (leading-zero hash prefix) and timestamp rules (strict increase from previous block, bounded future tolerance of 15,000 ms), with ValueErrors mapped to INVALID or FAILED status. Persistence tests updated to use deterministic incrementing timestamps.

Changes

Block Validation Rules

Layer / File(s)	Summary
PoW difficulty and timestamp validation minichain/chain.py	Adds time import; validates block hash against difficulty-based leading-zero prefix, enforces strictly increasing timestamps, and rejects timestamps more than 15,000 ms in the future.
Deterministic test timestamps tests/test_persistence.py	Updates block-building test helpers to set timestamps as previous block's timestamp plus 1000 instead of current wall-clock time.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested labels: Python Lang

Poem

A rabbit hops with pride so keen,
Each block now proves what work has been.
Time ticks forward, never back,
No future dates upon the track. ⏰
Hop, hop, hooray — the chain's secure! 🐇⛓️

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: adding PoW hash verification and timestamp validation to block checks.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/timestamp-pow-validation

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