fix(STX-340): use medium fee for gas fee in submission

[rarquevaux]

Explanation

Fix: Align bridge transaction gas price with validation

Problem

Users could submit bridge/swap transactions that fail with "insufficient funds" despite the UI showing sufficient balance. This occurred because:

• Validation (in bridge-controller) uses medium gas price estimates
• Submission (in bridge-status-controller) was using high gas price estimates

This mismatch allowed transactions to pass validation but fail immediately at submission time. In the STX case it times-out in Sentinel.

Example (uses a non STX transaction for simplicity but the same happens with STX)

A swap of ~0.002854 ETH → USDC on Base:

{
"quote": {
"srcTokenAmount": "2829027500000000",
"gasLimit": 726516,
"effectiveGas": 431877
}
}Submission failed with:

{
"error": {
"code": -32003,
"message": "insufficient funds for gas * price + value: have 2860507066881008 want 2864770358251460"
}
}The user had 0.00286 ETH but the transaction required 0.00286 ETH (4.26 gwei more than available) due to the higher high gas price at submission.

Fix

Changed bridge-status-controller to use medium gas estimates on submission (matching validation):

An alternate fix would be to use high on validation but it would prevent more swaps to go through.
NB: this happens in the non-gasless cases so irrespective of the fix we will see less and less occurrences of this issue as we release gasless swaps and auto-upgrade on all networks and clients.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

---

Note

Aligns gas estimation used at submission with validation to prevent mismatches (e.g., insufficient funds).

• In gas.ts, select maxFeePerGas/maxPriorityFeePerGas from txGasFeeEstimates[BRIDGE_PREFERRED_GAS_ESTIMATE] instead of hardcoded high
• Tests updated in gas.test.ts to validate preferred estimate usage, adjust fixtures to Medium, and improve missing-estimate handling
• Changelog updated under Unreleased to note the fix

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