PM-4686: block challenge activation for invalid billing accounts

[jmgasper]

What was broken
Challenges tied to projects could still be activated when the linked billing account was inactive, expired, or had no remaining budget.

Root cause
Activation validation only checked whether a project billing account id existed. It did not verify billing-account lifecycle state or remaining funds against the billing account data.

What was changed
Added billing-account detail lookup support in the project helper, including normalized active, end-date, status, and remaining-budget fields.
Updated challenge activation validation to block project-backed launches when the billing account is missing, inactive, expired, not found, or has no remaining funds.
Wired the billing account validation into challenge status updates while preserving the existing project billing-account persistence flow.

Any added/updated tests
Added focused unit coverage for the new activation billing-account validation paths.
Updated challenge service tests to cover inactive, expired, insufficient-funds, and valid billing-account activation scenarios.
Validation notes: pnpm lint passed. pnpm build could not run because this repo has no build script. pnpm test still fails in pre-existing TimelineTemplateService tests unrelated to this change, while the focused billing-account unit tests pass.