fix: for postquote payments payment token for MM Pay transaction should not be reset when accountOverride is changed

[jpuri]

Explanation

For postquote payments payment token for MM Pay transaction should not be reset when accountOverride is changed. This is ensure on UI token selection is not reset when account selection is changes.

References

Related to https://consensyssoftware.atlassian.net/browse/CONF-1381

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

Low Risk
Low risk: small conditional change to when paymentToken is cleared, limited to post-quote flows and covered by a new unit test.

Overview
Prevents TransactionPayController.setTransactionConfig from clearing transactionData.paymentToken when accountOverride changes in post-quote flows (isPostQuote: true), avoiding UI token selection resets.

Adds a regression test covering the new behavior, and documents the fix in CHANGELOG.md.

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