feat(issues): Added state_reason, sub-issues fields (#543)

The GitHub LIST endpoint returns several fields currently not in the tap
schema. This PR adds them:

---

`state_reason`: The reason for the state change. Ignored unless state is
changed. [docs
link](https://docs.github.com/en/rest/issues/issues?apiVersion=2026-03-10#list-repository-issues)

Can be one of: `completed`, `not_planned`, `duplicate`, `reopened`,
`null`

---

`parent_issue_url`: URL to get the parent issue of this issue, if it is
a sub-issue. String or null. Format: URI. NOT documented as a response
to LIST endpoint but is documented in [sub-issue
docs](https://docs.github.com/en/rest/issues/sub-issues).

---

`sub_issues_summary`: object showing aggregated progress of an issue's
sub-issues (GitHub docs copilot description).

Fields:
 - total (integer) — total number of sub-issues
 - completed (integer) — number of completed sub-issues
 - percent_completed (integer) — percent complete (integer)

Also not documented as a response to LIST endpoint but is documented in
[sub-issue docs](https://docs.github.com/en/rest/issues/sub-issues).

Co-authored-by: Adam Rubinstein <adamrubinstein@Adams-MacBook-Pro.local>

Author: adamrubinstein-shopify

tap_github/repository_streams.py

@@ -976,6 +976,16 @@ def post_process(self, row: dict, context: Context | None = None) -> dict:
                 th.Property("patch_url", th.StringType),
             ),
         ),
+        th.Property("state_reason", th.StringType),
+        th.Property("parent_issue_url", th.StringType),
+        th.Property(
+            "sub_issues_summary",
+            th.ObjectType(
+                th.Property("total", th.IntegerType),
+                th.Property("completed", th.IntegerType),
+                th.Property("percent_completed", th.IntegerType),
+            ),
+        ),
     ).to_dict()