fix: align Context logging methods with MCP spec data type

[paikend]

Summary

• Change message: str → data: Any in Context.log() and convenience methods (debug, info, warning, error)
• Remove the extra parameter — callers can now pass structured data directly as data
• Aligns with ServerSession.send_log_message(data: Any) which already accepts Any

Motivation

The MCP specification defines log data as unknown (any JSON-serializable type), but Context.info() and friends only accepted str.

Before:

# Type error — spec allows this but SDK didn't
await ctx.info({"key": "value", "count": 42})

# Workaround via extra parameter
await ctx.log(level="info", message="msg", extra={"key": "value"})

After:

# Now works — any JSON-serializable type
await ctx.info({"key": "value", "count": 42})
await ctx.info("plain string still works")
await ctx.info([1, 2, 3])

Breaking Change

• message parameter renamed to data in log(), debug(), info(), warning(), error()
• extra parameter removed from log() and convenience methods

Callers using positional args (ctx.info("msg")) are unaffected. Callers using message= keyword or extra= need to update.

Test plan

• All 1143 existing tests pass (0 failures)
• Updated test_logging_callback.py to use new API
• test_context_logging in test_server.py passes (string data still works)

Fixes #397

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