Detect negative-weight edges in the payment graph checker

[Kingsman-99]

Description

PaymentGraphChecker validates the structure of payment graphs but does not detect edges with negative weights (amounts). A negative-weight edge can cause unbounded fund extraction when the graph is traversed greedily.

Acceptance Criteria

• checkGraph(graph) returns a validation failure when any edge has weight < 0
• The failure message names the offending edge (source to target)
• Valid graphs with all non-negative weights continue to pass
• Unit tests pass

Context

• Target file: src/graph/PaymentGraphChecker.ts
• Zero-weight edges are allowed (they represent pass-through hops)

[woahwhattheheck]

Submitted a PR for this: #830

Implementation note, since the issue references an API that does not exist yet:
PaymentGraphChecker currently models the graph as an unweighted
Map<string, Set<string>> and has no checkGraph, so the PR introduces the
weighted-edge types (PaymentGraphEdge, PaymentGraph) alongside
checkGraph(graph) rather than adding a check to an existing one.

Two choices I made that are easy to change if you'd rather they went the other
way:

• checkGraph accepts either { edges } or a bare PaymentGraphEdge[], as
  the issue did not pin the shape.
• Weights may be number or bigint. Amounts elsewhere in the SDK are
  bigint, but graph weights read naturally as plain numbers, so both are
  compared against the matching zero.

Zero-weight edges stay valid per the issue. 15/15 tests pass in
test/paymentGraphChecker.test.ts, and tsc --noEmit reports the same 210
errors as unmodified main with none in the file touched.

[woahwhattheheck]

I'd like to work on this — requesting assignment.

I should be straight about the order of events: I opened PR #830 for this
issue before asking. CONTRIBUTING is explicit that coding should not start
until a maintainer assigns the issue, and I did not read it before starting.
That is my mistake, not a grey area.

The PR stands on its own if it is useful to you, but I am not claiming any
standing from it. If you would rather assign this to someone else, say so and
I will close it — no need to explain.

If you do assign me, I will re-push under your branch convention as
feat/issue-773-negative-edge-weights; my current branch omits the issue- prefix.

[rupesh-kumar-sah]

Hello @maintainers,

I would like to resolve this issue. Here is my proposed execution plan:

• Implementation: Deliver a clean, modular solution for Detect negative-weight edges in the payment graph checker strictly following Stellar-split/split-sdk conventions.
• Testing: Cover changes with automated tests ensuring CI passes smoothly.

Happy to pick this up today—please assign to me!