Honor IncludeCategory in the JSON log format

[TemRevil]

Fixes #2469

Problem

With Amazon.Lambda.Logging.AspNetCore, the ILogger category is written on every line in text format when LambdaLoggerOptions.IncludeCategory is true (which is the default). The JSON branch of LambdaILogger.Log, however, only forwards the message template and its arguments and never emits the category, so IncludeCategory is silently ignored in JSON mode. Migrating a function from text to JSON logging drops the category entirely, with no way to opt back in.

Change

When IncludeCategory is set, the JSON branch now prepends a {Category} placeholder to the message template and supplies the category as the matching argument. Because the JSON formatter turns named template parameters into top-level properties, the category surfaces as a queryable property (and in the rendered message), matching the parity the text format already provides.

• Text format: unchanged.
• JSON format with IncludeCategory = false: unchanged (no extra property, same parameter count).
• JSON format with IncludeCategory = true: the category is included.

This follows the reporter's suggested "at minimum honor IncludeCategory" approach and needs no runtime-side change. If the maintainers would rather expose it as a dedicated top-level category field emitted by JsonLogMessageFormatter (the issue's preferred option), I'm happy to take that direction instead.

Testing

Amazon.Lambda.Logging.AspNetCore.Tests on net10.0: Passed! Failed: 0, Passed: 16. Added TestJSONParameterLoggingIncludesCategoryWhenEnabled (category present, parameter count goes 3 to 4) and TestJSONParameterLoggingOmitsCategoryWhenDisabled (no category, count stays 3); the existing TestJSONParameterLogging still passes since it uses IncludeCategory = false.