fix: validate negative TAO amounts in staking

[OpenSourceSoul]

Problem
StakeParams::new_tao() accepted negative TAO amounts without validation. When a negative f64 is cast to u64, it produces a massive value (close to u64::MAX), causing incorrect staking amounts.

Solution

• Add validation to reject negative amounts in new_tao()
• Return Result<Self, BittensorError> for proper error handling
• Added tests for negative amount rejection and zero amount handling

Changes

• bittensor-rs/src/extrinsics/staking.rs: 48 insertions, 20 deletions

This prevents users from accidentally staking massive amounts when passing negative values.

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Staking operations now include enhanced input validation to prevent processing of negative amounts, providing clear error messages for invalid inputs.

• Tests

  • Added comprehensive test coverage for edge cases, including zero amounts and negative amount rejection.

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[coderabbitai]

Walkthrough

The StakeParams::new_tao method is refactored to return a Result<Self, BittensorError> instead of Self, introducing validation to reject negative amounts. Error handling is updated at call sites, documentation examples are modified to reflect the new behavior, and tests are added to verify zero amounts are permitted while negative amounts are properly rejected.

Changes

Cohort / File(s)	Summary
Staking validation and error handling bittensor-rs/src/extrinsics/staking.rs	Refactored StakeParams::new_tao to return Result<Self, BittensorError> with validation that rejects negative amounts. Updated call sites to handle the Result type, modified documentation examples to use error-handling patterns, and added tests for zero amounts (allowed) and negative amounts (rejected with error).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• Issue #8: Directly addresses the same code-level fix to StakeParams::new_tao — adding non-negative validation, changing return type to Result, and introducing corresponding tests.

Poem

🐰 A stake must be worthy, never less than the ground,
No negative values in validation we've found,
Result types and Errors now guard the path true,
Each tao will validate before passing on through! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding validation for negative TAO amounts in the staking module, which is the core objective of this pull request.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[OpenSourceSoul]

hi! @distributedstatemachine check please