feat(euclid): default fallback for transactions

[prajjwalkumar17]

Description

This PR introduces support for a new routing algorithm variant: DefaultFallback. The purpose of this routing type is to provide a simple, last-resort fallback mechanism when no other routing logic matches or is applicable.

The following updates are included:

• Added a new variant DefaultFallback in the RoutingType, Output, and StaticRoutingAlgorithm enums.
• Implemented logic in the Euclid interpreter to handle DefaultFallback by returning the provided connectors directly as eligible connectors.
• Integrated evaluation support in the routing_evaluate handler.
• Updated format_output to support serialization of DefaultFallback for API responses.
• Adjusted rule validation utility to allow DefaultFallback algorithms to pass without evaluation.

Outcomes

• Enables merchant configurations to specify a static fallback connector list to be used when advanced routing fails or yields no result.
• Helps improve system reliability and guarantees at least one connector path is available.
• Keeps evaluation logic modular and easily extensible.

Diff Hunk Explanation

src/euclid/ast.rs

• Added DefaultFallback as a new variant in RoutingType and Output enums.

src/euclid/types.rs

• Added DefaultFallback(Vec<ConnectorInfo>) to the StaticRoutingAlgorithm enum.

src/euclid/handlers/routing_rules.rs

• Handled routing evaluation logic for the new DefaultFallback algorithm.
• Updated the output formatting (format_output) for DefaultFallback.

src/euclid/interpreter.rs

• Implemented direct passthrough logic for evaluate_output when using DefaultFallback.

src/euclid/utils.rs

• Updated validate_routing_rule to accept DefaultFallback as a valid static routing configuration.

This PR is linked to Issue #101 and provides the implementation required to resolve it.

[Copilot]

Pull Request Overview

Adds a new DefaultFallback routing algorithm to provide a fallback set of connectors for transactions.
Key changes:

• Introduced DefaultFallback(Vec<ConnectorInfo>) in StaticRoutingAlgorithm, AST enums, and Output.
• Updated validate_routing_rule and evaluate_output to handle the new variant.
• Enhanced the routing handler and output formatter to support DefaultFallback.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/euclid/utils.rs	Accept DefaultFallback in validate_routing_rule
src/euclid/types.rs	Added DefaultFallback variant to StaticRoutingAlgorithm
src/euclid/interpreter.rs	Implemented evaluate_output arm for DefaultFallback
src/euclid/handlers/routing_rules.rs	Handled DefaultFallback in routing_evaluate and JSON
src/euclid/ast.rs	Included DefaultFallback in RoutingType and Output

Comments suppressed due to low confidence (3)

src/euclid/handlers/routing_rules.rs:648

• There are no existing tests verifying the behavior of the new DefaultFallback in routing_evaluate or format_output. Please add unit tests to cover both success and failure paths for this variant.

        Output::DefaultFallback(connectors) => {

src/euclid/types.rs:50

• Public API surface was extended with DefaultFallback. Consider adding or updating doc comments in this enum (and any user-facing docs) to explain when and how this fallback is applied.

    DefaultFallback(Vec<ConnectorInfo>),

src/euclid/handlers/routing_rules.rs:591

• [nitpick] The error context only includes the algorithm’s to_string() output. It may be more helpful to include the connector list or a descriptive message (e.g., number of fallback connectors) to aid debugging.

                    EuclidErrors::FailedToEvaluateOutput(format!(

[Copilot]

Cloning the connectors vector twice causes two allocations. You can clone once and reuse the result, e.g. let cloned = connectors.clone(); Ok((cloned.clone(), cloned)) to reduce overhead.

Suggested change

	Output::DefaultFallback(connectors) => Ok((connectors.clone(), connectors.to_vec())),
	Output::DefaultFallback(connectors) => {
	let cloned = connectors.clone();
	Ok((cloned.clone(), cloned))
	},

[Copilot]

The DefaultFallback variant is always considered valid, even if its connector list is empty. Consider adding a check to ensure the fallback list is non-empty to prevent downstream runtime errors.

Suggested change

	| StaticRoutingAlgorithm::DefaultFallback(_)
	| StaticRoutingAlgorithm::DefaultFallback(connectors) => {
	if connectors.is_empty() {
	return Err(EuclidErrors::InvalidRequest(
	"DefaultFallback connector list cannot be empty".to_string(),
	)
	.into());
	}
	Ok(())
	}