fix: support file and stdin JSON inputs

[GannonSun]

Summary

• support --params @file.json and --params - for the raw api command and generated service commands
• support --data @file.json and --data - on the same paths
• centralize structured JSON input resolution, update help text, and add coverage for file/stdin inputs

Why

On Windows PowerShell, inline JSON like --params '{"foo":"bar"}' can reach native executables with the inner double quotes stripped, so the CLI receives {foo:bar} instead of valid JSON.

That means the failure happens before request construction, and the CLI cannot reliably reconstruct the lost quoting. This change takes the smallest reliable path by adding deterministic cross-platform alternatives: @file and stdin (-).

Validation

• go test ./internal/cmdutil ./cmd/api ./cmd/service
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.1.6 run --new-from-rev=origin/main

Notes

• go test -race -gcflags="all=-N -l" -count=1 ./cmd/... ./internal/... ./shortcuts/... could not be run in this Windows environment because gcc/cgo is not installed.
• A broader go test -count=1 ./cmd/... ./internal/... ./shortcuts/... run on Windows hit existing unrelated failures in packages such as cmd/auth, internal/validate, and shortcuts/mail/draft.

Refs #64

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