feat(fixtures): accept multiple fixture files (closes #910)

[0xdeadd]

Closes #910.

The fixtures command previously required exactly one file argument. This extends it to accept one or more files, executed sequentially in the order given.

stripe fixtures ./customer.json ./subscription.json ./teardown.json

Behavior

• UpdateEnv is called after each fixture, so later files can reference vars produced by earlier ones via ${.env:VAR}.
• Execution aborts on the first error rather than continuing, to preserve the established single-file behavior and surface failures immediately. Users who want try-each semantics can chain at the shell level.
• The existing single-file invocation continues to work unchanged.

Implementation

• Added a MinimumNArgs validator to pkg/validators/cmds.go, mirroring the existing MaximumNArgs, since the package only had Exact and Maximum variants. Includes tests.
• Switched the fixtures command from validators.ExactArgs(1) to validators.MinimumNArgs(1) and wrapped the runner body in a per-file loop.
• Updated the command's Use and Long to reflect the new shape.

Test

$ go test ./pkg/validators/... ./pkg/fixtures/...
ok  	github.com/stripe/stripe-cli/pkg/validators	0.204s
ok  	github.com/stripe/stripe-cli/pkg/fixtures	0.283s

gofmt and go vet clean.

[cla-assistant]

All committers have signed the CLA.