feat(new_page): support incognito browser contexts

[dhananjay6561]

solves #1985

Summary

This PR adds incognito support to the new_page tool by introducing an optional incognito boolean parameter.

When incognito: true is provided, new_page creates a fresh isolated browser context for that page. This improves session isolation for multi-user and auth-sensitive testing scenarios.

Changes

• Added incognito?: boolean to new_page tool schema and context API.
• Implemented incognito page creation in McpContext.newPage(...) using a fresh browser context per call.
• Added validation to reject using incognito and isolatedContext together.
• Updated CLI command metadata for new_page to include incognito.
• Added tool tests covering:
  • incognito context creation,
  • new context per incognito page,
  • mutual-exclusivity validation with isolatedContext.

Notes

• Existing isolatedContext behavior remains unchanged.
• This provides browser-context isolation; it does not launch a separate Chrome process/profile.

[OrKoN]

This does not solve #1985 because this does not add any new functionality compared to isolated contexts.