fix(ipc): trim leading whitespace in StreamCodec decode

[meetrick]

Summary

Trim leading whitespace from decoded JSON-RPC messages in StreamCodec, resolving a TODO and improving IPC message handling.

Problem

When StreamCodec::decode() parses JSON-RPC messages over IPC, it includes leading whitespace (newlines, tabs, spaces) that appears between messages. This is unnecessary since:

1. Leading whitespace is not part of the JSON message
2. It wastes memory storing meaningless bytes
3. It clutters debug logs with extra whitespace
4. The start_idx variable already tracks where JSON actually begins, but was not being used to discard prior bytes

Before

Input:  "{ msg: 1 }\n\n\n\n{ msg: 2 }"
Output: "\n\n\n\n{ msg: 2 }"  // includes leading whitespace

After

Input:  "{ msg: 1 }\n\n\n\n{ msg: 2 }"
Output: "{ msg: 2 }"  // clean JSON only

User Benefits

• Cleaner logs: Debug output shows clean JSON without leading garbage
• Memory efficiency: No allocation for unnecessary whitespace bytes
• Predictable output: Messages always start with { or [

Changes

• stream_codec.rs: Discard bytes before start_idx in decode()
• Updated whitespace test expectations
• Updated huge test to start with { directly
• Removed TODO comment