test(core): add unit tests for charge() failure paths

[Demilade10]

Description

Add unit tests covering charge() failure scenarios that are not yet tested, so regressions in error handling are caught automatically instead of relying on manual testing.

Requirements and Context

• The happy path (create_plan -> approve -> subscribe -> charge) is already verified end-to-end on testnet
• Failure paths are currently untested and need explicit coverage
• Tests must use the existing test env setup, mock auth, and token contract setup patterns already present in the test module
• allowance expiration_ledger is capped at 3,000,000 ledgers - useful for constructing the expired allowance test case

Suggested Execution

Branch name: test/charge-failure-paths

Files to touch:

• src/test.rs (or wherever the existing test module lives - add new test functions)

Example commit message: test(core): add unit tests for charge failure paths

Test and Commit

• Test: charge() fails when allowance is insufficient
• Test: charge() fails when allowance has expired (past expiration_ledger)
• Test: charge() fails when called before the billing interval has elapsed
• Test: charge() fails when subscription has been cancelled
• Test: charge() fails when called by an unauthorized address
• All new and existing tests pass with cargo test
• Tests follow existing naming and structure conventions in the test module
• Include test output in the PR

Guidelines

• Assignment required before starting - comment to request assignment
• PR description must include: Closes test(core): add unit tests for charge() failure paths #2

Complexity

Trivial - 100 points