CLI gen: detect webhook-shaped sub-objects and emit composite --webhook-* builder

Part of #338. Depends on gap #1 (per-parameter flags).

Today

Many APIs have a webhook config nested inside the request body:

webhook:
  type: object
  properties:
    url: { type: string, format: uri }
    headers: { type: object, additionalProperties: { type: string } }
    metadata: { type: object }
    events: { type: array, items: { type: string, enum: [...] } }

Per-property flattening (gap #1) skips nested objects today. Firecrawl hand-writes BuildCrawlWebhookAsync / BuildBatchWebhookAsync to compose flags like --webhook-url, --webhook-header k=v, --webhook-metadata k=v, --webhook-event ... into a single webhook struct. ~150 lines, repeated twice with cosmetic differences.

Target

firecrawl crawl https://example.com \
  --webhook-url https://my.app/hook \
  --webhook-header authorization=Bearer\\ xyz \
  --webhook-header x-request-id=abc \
  --webhook-metadata customer-id=42 \
  --webhook-event completed --webhook-event errored

Proposed approach

1. Detection heuristic: a nested object property is webhook-shaped when it has:
   • a property of string/format: uri (any name; url/endpoint/callback/webhook preferred)
   • AT LEAST ONE of: a string-map (headers), a generic-object (metadata), an enum-array (events)
2. Generate --<prefix>-* flag set where <prefix> is the property's kebab-cased name (webhook, notification-url, etc.):
   • URL → single --<prefix>-url (string)
   • headers map → repeatable --<prefix>-header KEY=VALUE (Option<Dictionary<string,string>> with custom parser)
   • metadata map → repeatable --<prefix>-metadata KEY=VALUE
   • events array → repeatable --<prefix>-event with FromAmong(...) for enum constraint
3. Composite builder in generated invocation: when --<prefix>-url is present, construct the nested object from all --<prefix>-* flags and assign to the request body.
4. Add helpers to CliRuntime: ParseKeyValueAsync(IEnumerable<string> tokens) -> Dictionary<string,string>.
5. Vendor extension x-cli-webhook: false to opt out (fall back to nested --<prop>-json).

Acceptance criteria

• Firecrawl crawl and batch-scrape expose --webhook-url, --webhook-header, --webhook-metadata, --webhook-event after regen.
• Repeated flags accumulate (--webhook-header a=1 --webhook-header b=2).
• Without any --webhook-* flag, the webhook field stays unset in the request.
• Snapshot test covers an operation with a webhook-shaped property.

Start by tracing CLI generation for Firecrawl's crawl and batch-scrape operations, then inspect the generated invocation and the CliRuntime helpers. Implement the webhook-shaped flag set and composite assignment described in the issue, including opt-out handling. Regenerate and verify repeated flags, the unset case, and a snapshot test for a webhook-shaped property.