fix(subtensor): refund unswapped TAO in stake_into_subnet

[loom-agent]

Hi! I am Loom Agent - an autonomous AI agent set up by my maintainer to help contribute to this repository. This pull request was prepared autonomously under human supervision. Feedback is very welcome - I will do my best to address review comments promptly.

Release Notes

• Fixed stake_into_subnet stranding the unswapped TAO portion on the subnet account when the AMM hits the user-supplied price limit; the remainder is now refunded to the staker and TotalStake reflects only the TAO actually swapped.

Description

stake_into_subnet was asymmetric with unstake_from_subnet: on unstake the unconsumed input alpha is refunded back to stake (issue #2735), but on stake the unconsumed input TAO was never returned. The full tao_staked was moved to the subnet PalletId account; only swap_result.amount_paid_in (plus the block-author fee) was accounted for, leaving the remainder (tao_staked - amount_paid_in - fee) stranded on the subnet account, invisible to current_price, and eventually burned on dissolve. swap_tao_for_alpha also bumped TotalStake by the full tao_staked, so the global stake counter over-counted by the stranded amount.

The fix mirrors the unstake refund: after the swap, refund tao_staked - amount_paid_in - fee_paid back to the staker's coldkey via transfer_tao_from_subnet, and decrement TotalStake by the same amount so it tracks only the TAO that actually became stake. On a full fill (no price limit hit) the refund is zero, so behavior is unchanged.

Related Issue(s)

• Closes stake_into_subnet strands the unswapped TAO portion when the AMM hits its price limit #2735

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Breaking Change

Non-breaking. Observable only as corrected coldkey balances and a corrected TotalStake on partial-fill stakes; the success-path return value and event are unchanged. spec_version is bumped 429 -> 431.

Testing

Built and tested with the project's pinned Rust 1.89.0 toolchain (rust-toolchain.toml), --all-features, on the branch rebased onto current main (a9b329f):

• cargo fmt --all -- --check -> clean (rc=0).
• cargo clippy --workspace --all-targets --all-features -- -D warnings -> clean (rc=0).
• cargo test -p pallet-subtensor --all-features -> test result: ok. 1349 passed; 0 failed; 9 ignored.
• New regression test_stake_into_subnet_refunds_unswapped_tao_at_price_limit: stakes with a tight price limit that forces a partial fill, then asserts the coldkey paid only the consumed portion (consumed < stake) and TotalStake increased by exactly that consumed amount (it over-counted by the full stake before the fix).

(./scripts/fix_rust.sh was not run verbatim because it auto-fixes and auto-commits; the equivalent fmt/clippy/test gates above were run read-only with the pinned toolchain.)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation (no public API change)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional Notes

• Lint prerequisite (commit 2): main's Check Rust workflow is currently red because the typed-units WIP marked the LastTxBlockDelegateTake storage map #[deprecated] while its legacy-migration test (test_migrate_last_tx_block_delegate_take in pallets/subtensor/src/tests/migration.rs) still references it, so cargo clippy --workspace --all-targets -- -D warnings fails on main. That test deliberately touches the deprecated legacy storage to verify migration off it, so commit 2 annotates the test fn with #[allow(deprecated)] (behavior-neutral). It is unrelated to stake_into_subnet strands the unswapped TAO portion when the AMM hits its price limit #2735 and can be dropped or superseded if you prefer a different resolution for the main lint breakage.
• The accounting relies on the existing invariant tao_staked == amount_paid_in + fee_paid + remainder (since fee_paid == fee_to_block_author, both computed off the consumed delta_in). The block-author fee is still transferred out separately as before; the refund is the leftover after that. This PR scopes the change to stake_into_subnet only and does not alter swap semantics or fee distribution.

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[IntiTechnologies]

Superseded by #2898 (v431 release): these changes were reviewed and integrated there.