feat(subtensor): split InsufficientBalance into TAO and Alpha variants

[loom-agent]

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Release Notes

• Changed the shared InsufficientBalance dispatch error into two distinct errors, InsufficientTaoBalance (TAO) and InsufficientAlphaBalance (Alpha stake), so SDK consumers and callers can tell which asset was insufficient. Chain-extension ABI is unchanged (both surface as code 12).

Problem

The single Error::InsufficientBalance was returned for both TAO balance checks and Alpha stake checks, so a caller that hit it could not determine which asset was short without extra probing (Closes #1960).

Root cause

pallets/subtensor/src/macros/errors.rs defined one shared InsufficientBalance variant. All TAO-side call sites in coinbase/tao.rs and the Alpha-side call site in BalanceOps::decrease_stake routed through the same error, losing the asset distinction.

The fix

• The existing variant is renamed in place to InsufficientTaoBalance (its enum position is preserved, so the numeric indices of the variants after it do not shift).
• A new InsufficientAlphaBalance variant is appended for the one Alpha-balance call site, BalanceOps::decrease_stake (pallets/subtensor/src/lib.rs).
• The four TAO-side call sites in coinbase/tao.rs now use InsufficientTaoBalance.
• The chain-extension error mapping (chain-extensions/src/types.rs) maps both new names to the existing Output::InsufficientBalance ABI code (12).
• spec_version bumped from 429 to 430.

Notes

• Prior PR feat(subtensor): split InsufficientBalance into TAO and Alpha variants #2883 was prematurely closed with a claim it was superseded by v431 release #2898, but v431 release #2898 was also closed without merge. The underlying issue Separate pallets/subtensor/src/macros/errors.rs:InsufficientBalance to two type of errors. #1960 remains unresolved.
• transaction_version is intentionally unchanged (1) per repo convention.
• Differs from prior attempt Fix: Issue #1960 - Refactor InsufficientBalance error #2325 (closed, no review): this preserves error indices in place and maps both errors to the existing chain-extension ABI code.

Testing

• UNTESTED: no build environment is available. The branch previously passed CI (rust checks green) on the original PR feat(subtensor): split InsufficientBalance into TAO and Alpha variants #2883.

Closes #1960

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