SK-2681: Add dict context support for Conditional Data Access

[saileshwar-skyflow]

Summary

• Add dict support for the ctx parameter in bearer token and signed data token generation
• Add _validate_and_resolve_ctx() with key validation (alphanumeric + underscores only for CEL compatibility)
• Add error messages for invalid ctx type and invalid dict keys
• Add 14+ unit tests for the new validation function
• Update README with string vs dict context documentation and CEL expression examples
• Update sample files with both string and dict context approaches

Test plan

• _validate_and_resolve_ctx() manually verified: string, dict, None, empty string, invalid type, invalid keys, nested objects
• Pre-existing test collection error on main (requires credentials.json — not a regression)
• End-to-end test with a Skyflow vault using a Conditional Data Access policy

Refs SK-2681, DOCU-1440