Use jsonc language tag for JSON examples containing comments

Three code blocks are tagged ```json but contain comments, which JSON
does not permit, so the examples cannot be parsed or copied as-is:

- copilot/reference/hooks-reference.md uses a /* hook entries */ placeholder
- copilot/how-tos/copilot-sdk/troubleshooting/mcp-debugging.md uses a
  leading // comment
- copilot/reference/copilot-usage-metrics/lines-of-code-metrics.md uses
  three trailing // comments

Retag these fences as ```jsonc, matching the convention already used
elsewhere in the content directory for JSON with comments (for example
the dev container examples in codespaces/). The renderer registers
'jsonc' as an alias of 'json' in src/content-render/unified/processor.ts,
so highlighting on docs.github.com is unchanged while the fences also
render correctly on github.com.

The comments are left in place; only the language tags change.

Author: Prajwal-chikkur07

content/copilot/how-tos/copilot-sdk/troubleshooting/mcp-debugging.md

@@ -392,7 +392,7 @@ This provides a web UI to:
 
 Check your server supports the protocol version the SDK uses:
 
-```json
+```jsonc
 // In initialize response, check protocolVersion
 {"result":{"protocolVersion":"2024-11-05",...}}
 ```

content/copilot/reference/copilot-usage-metrics/lines-of-code-metrics.md

@@ -67,7 +67,7 @@ Unlike completions or chat, {% data variables.copilot.copilot_agent_short %} doe
 
 Example output for agent-related LoC activity:
 
-```json
+```jsonc
 "totals_by_language_feature": [
   {
     "language": "unknown",

content/copilot/reference/hooks-reference.md

@@ -841,7 +841,7 @@ Use `disableAllHooks` when you want to keep your hook configuration on disk but
 
 Set `disableAllHooks` to `true` at the top level to skip every hook in the file without deleting it.
 
-```json
+```jsonc
 {
   "version": 1,
   "disableAllHooks": false,