protocol_connect: centralize base64 encoding/decoding logic

[tsubakiky]

This pull request introduces improvements to the Connect protocol implementation, focusing on enhanced base64 handling, thread safety, default port configuration, and code readability. The most significant changes include the addition of utility functions for base64 encoding/decoding with padding, thread-safe caching in error handling, and the introduction of a default HTTP port constant.

Base64 Handling Enhancements:

• src/connect/protocol_connect/base64_utils.py: Added utility functions for base64 encoding/decoding, including decode_base64_with_padding, decode_urlsafe_base64_with_padding, and encode_base64_without_padding, to handle padding and improve code reuse.
• src/connect/protocol_connect/connect_handler.py: Replaced direct base64 decoding with decode_urlsafe_base64_with_padding for better readability and consistency.
• src/connect/protocol_connect/error_json.py: Updated error serialization/deserialization to use the new base64 utility functions. [1] [2]

Thread-Safe Caching:

• src/connect/protocol_connect/error_json.py: Implemented thread-safe initialization for the _string_to_code cache using a double-checked locking pattern to ensure safe concurrent access. [1] [2]

Default Port Configuration:

• src/connect/protocol_connect/constants.py: Added DEFAULT_HTTP_PORT constant for cleaner and centralized default port handling.
• src/connect/protocol_connect/connect_client.py: Updated default port handling in Address initialization to use DEFAULT_HTTP_PORT.

Code Readability Improvements:

• src/connect/protocol_connect/end_stream.py: Refactored metadata and error handling for clarity by introducing intermediate variables and simplifying logic.
• src/connect/protocol_connect/marshaler.py: Simplified URL query parameter updates by reusing encoded data in base64 operations.

Miscellaneous Fixes:

• src/connect/protocol_connect/connect_handler.py: Corrected a typo in the module docstring ("ConnectHander" → "ConnectHandler").
• src/connect/protocol_connect/constants.py: Improved user agent string formatting to include only the major and minor Python version for brevity.