[copybara-service]

fix(cli): validate --model on intents; add --stdin, -f/--file, and --system

Bring the generated Create intents (generate, image, video, music) and agent run in line with the hand-written porcelain (tts, analyze, transcribe) on --model validation and text/media input handling:

• --model validation:
  • Hold an explicit --model override on generate, image, video, music, and agent run to the single-segment model ID contract (normalizeModelFlag), rejecting malformed model IDs with CLI_VALIDATION (exit 2) before any request and stripping a models/ prefix in-place. Unset or blank flags fall back to the command's default.
• --stdin and -f/--file text input:
  • Add --stdin to generate, image, video, music, and analyze to read stdin to EOF as prompt text rather than a JSON request body. Add -f/--file to music (generate, image, video, and analyze already accept text files via -i/--input).
  • When supplied alone, the text becomes the prompt; alongside a positional prompt (or analyze question), it is placed as a text content block ahead of the positional prompt and after any -i media, and merges into --body input.
  • Reject --stdin combined with --body @-, --stdin combined with -f, empty stdin, or unreadable files as CLI_VALIDATION (exit 2).
  • Without --stdin, piped stdin remains the JSON request body; when piped stdin fails JSON unmarshaling on commands supporting --stdin, hint to pass --stdin for prompt text.
  • Update the README.md command table to list --stdin, -f/--file, and --system, and remove the unimplemented -f/--file entry from analyze.
• Request-input registration, --resume precedence, and analyze positional media hint:
  • Mark -i/--input, --mime-type, -f/--file, and --stdin as request inputs (flagutil.MarkRequestInput) in attachIntentInputs so an open, silent stdin pipe does not stall for 1s when input is already supplied via -i or -f.
  • Check flagutil.IsAsyncResume in attachIntentInputs before resolving -i media or reading --stdin so video --resume <id> immediately reports conflicting create flags (--input, --stdin, --model) without network calls or stdin reads.
  • When analyze is invoked without --input or --stdin and the first positional argument is media (classifyMediaInput), report that it is media rather than a question and hint gemini-api analyze -i <path> [question].
• --system on generate (OAD overlay & regenerated CLI):
  • Declare system ($.system_instruction) on the generate intent in overlays/cli/commands.yaml.
  • Trim one redundant flag summary line from the image and video command descriptions in overlays/cli/commands.yaml so adding --stdin keeps image --help and video --help within the 40-line help budget.
  • Regenerate specs/genai/openapi_cli.with-overlays.json, internal/cli/agent/intent_{generate,image,video}.go, and internal/usage/schema.go.
• Tests:
  • Add contract tests in tests/contract/intent_inputs_test.go, tests/contract/porcelain_inputs_test.go, tests/contract/async_contract_test.go, and tests/contract/stdin_deadline_test.go.