feat(core): add surcharge support for MIT

[swangi-kumari]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/router/src/core/payments.rs	22% smaller
Dockerfile	Unsupported file format
crates/hyperswitch_domain_models/src/payment_method_data.rs	0% smaller

[XyneSpaces]

Automated Review Summary

PR: feat(core): add surcharge support for MIT
Reviewer: PR Reviewer Agent

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Findings

⚠️ 1. Best-effort surcharge calculation silently swallows errors

File: crates/router/src/core/payments.rs

calculate_mit_external_surcharge returns Option and logs warnings for all error paths. While the "best-effort" semantics are documented, consider if any failure modes should surface to the caller rather than being silently ignored. For example:

• MCA fetch failure (configuration error that should alert operators)
• UCS timeout (may indicate service degradation)

Current behavior is acceptable for recurring payment UX, but ensure monitoring is in place for surcharge calculation failure rates.

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🔍 2. Missing instrumentation on new async functions

Files: crates/router/src/core/payments.rs

The new async functions run_external_surcharge_ucs and calculate_mit_external_surcharge should have #[instrument(skip_all)] attributes for observability, consistent with other payment core functions.

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🔍 3. Card IIN extraction fallback chain

File: crates/router/src/core/payments.rs

The fallback from payment_method_data.get_card_iin() to payment_attempt.payment_method_data JSON extraction is reasonable, but document why PSP-tokenized MIT flows don't have a Card variant available. This is subtle behavior that future maintainers may question.

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Verdict: ✅ Approve with minor suggestions — Clean refactor with proper best-effort semantics for MIT surcharge. Add instrumentation attributes before merge.