feat: Derive fiat asset from feature flags before hardcoded fallback

[OGPoyraz]

Explanation

Currently deriveFiatAssetForFiatPayment resolves the fiat asset for a payment entirely from a hardcoded map (FIAT_ASSET_ID_BY_TX_TYPE). This makes it impossible to adjust asset mappings without a code release.

This PR introduces a 3-tier resolution for fiat assets:

1. Feature flag - reads from confirmations_pay_fiat.assetPerTransactionType[txType] via RemoteFeatureFlagController
2. Hardcoded map - falls back to the existing FIAT_ASSET_ID_BY_TX_TYPE constant
3. ETH on mainnet - terminal fallback when neither source has an entry

The function signature gains a messenger parameter (already available at the call site in fiat-quotes.ts), and the return type tightens from TransactionPayFiatAsset | undefined to TransactionPayFiatAsset since the ETH mainnet fallback guarantees a value.

References

• Feature flag key: confirmations_pay_fiat

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

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Note

Medium Risk
Changes runtime fiat quote behavior and introduces a new remote-flag-controlled override plus a mainnet-ETH default, which could affect pricing/quoting for previously-unmapped transaction types if flags are misconfigured.

Overview
Updates fiat quote asset selection to be feature-flag driven: deriveFiatAssetForFiatPayment now reads confirmations_pay_fiat.assetPerTransactionType[txType] (including batch txs via first nested type), then falls back to FIAT_ASSET_ID_BY_TX_TYPE, and finally to a new ETH_MAINNET_FIAT_ASSET default.

This changes the function signature to require a messenger, updates getFiatQuotes to use it, removes the “missing fiat asset mapping returns []” path, and adds unit tests plus a new getFiatAssetPerTransactionType helper in feature-flags.

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