Add ValidationOptions for lenient output schema validation

[lorenss-m]

Motivation and Context

Currently, MCP clients raise RuntimeError when tools don't return structured content matching their outputSchema. This breaks integrations with servers that have schema/implementation mismatches.

This PR adds optional lenient validation that converts these errors to warnings, allowing clients to continue operating with imperfect servers.

How Has This Been Tested?

• Added test suite in tests/client/test_validation_options.py
• All existing tests pass
• Tested with real MCP servers exhibiting validation issues

Breaking Changes

None. Default behavior unchanged. Lenient validation is opt-in:

from mcp.client.session import ValidationOptions

# Opt-in to lenient validation
options = ValidationOptions(strict_output_validation=False)
session = ClientSession(read, write, validation_options=options)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• Added ValidationOptions class following existing patterns (e.g., TransportSecuritySettings)
• Modified ClientSession.call_tool() to check options before raising validation errors
• Fixed workspace config issue with structured_output_lowlevel.py by creating proper package structure