fix(tax): guard division-by-zero panic in community-tax split adjustment

[GeoffreySHD]

Description (severity: chain-halt risk reachable via governance)

ProcessTaxSplits in x/tax/keeper/tax_split.go computes the community-tax adjustment as:

applyCommunityTax := communityTax.Mul(oracleSplitRate.Quo(communityTax.Mul(oracleSplitRate).Add(sdkmath.LegacyOneDec()).Sub(communityTax)))

The divisor is communityTax·(1 − oracleSplitRate). Both inputs are ordinary governance-settable parameters:

• communityTax = 1.0 is accepted by the SDK's distribution param validation,
• oracleSplitRate = 0 is accepted by validateOraceSplit (only rejects negatives and >1).

With that combination the denominator is zero and the LegacyDec division panics — inside ProcessTaxSplits, which runs within every taxable transaction (bank sends, market swaps, wasm calls through the tax handler wrappers). A single passed governance proposal setting those two values would make every taxable transaction panic → chain halt until an emergency upgrade corrects the parameter.

Reproduction (exact original expression, governing inputs): panics with division by zero.

Fix

Extract the computation into a pure, testable helper types.CommunityTaxAdjustment that returns the unadjusted communityTax whenever the numerator or denominator is non-positive. Behavior is identical for every previously-working parameter combination; only the panicking configuration changes (no community-tax share is taken — consistent with that configuration's semantics).

Testing

• New subtests in x/tax/types/compute_test.go (main's existing ComputeTaxes tests are preserved) covering the hand-computed normal case, zero inputs, and the previously panicking configuration (NotPanics).
• Full x/tax/... suite passes; golangci-lint (pinned v2.1.6) reports 0 issues on the canonical tree.

[GeoffreySHD]

Cherry-picked the osmosis heighliner image fix (0a6ee0b) — the earlier test-ibc-pfm red check was the repo-wide dead ghcr.io/strangelove-ventures image path (see #667), unrelated to this change.

[GeoffreySHD]

Friendly bump — fully green (31/31), e2e matrix included. This closes a governance-reachable chain-halt: in ProcessTaxSplits, the community-tax adjustment divides by communityTax*(1-oracleSplit), which is zero when governance sets communityTax=1.0 and oracleSplit=0 (both individually valid) — every taxable transaction would panic. The PR reproduces the panic on the exact original expression and guards only the divisor, preserving bit-identical behavior for every working configuration. Heads-up for the MM2 PR (#664): the redirect block lands in the same function, so on rebase keep the guarded helper. Would appreciate a review from @StrathCole @fragwuerdig @hoank101.