fix: add platform field and context_line extraction

[kashishhora]

Summary

Added

• platform field to ErrorData type (set to "javascript")
• context_line field to StackFrame type for capturing source code lines
• addContextToFrame function to extract code context from source files
• Comprehensive tests for context line extraction and platform field
• Protocol validation error tests

Modified

• captureException now sets platform: "javascript" on all error data
• Stack trace parsing now extracts context lines for user code (in_app: true)
• Context extraction only runs for application code to optimize performance

Test Plan

• Verify platform field is set to "javascript" on all captured errors
• Verify context_line is captured for user code frames
• Verify context_line is NOT captured for library/node_modules frames
• Verify graceful handling when source files cannot be read
• Run test suite to confirm all new tests pass