fix malformed URL causing 'since' filter to be dropped for Issues and Comments streams

[leslievandemark]

Description of change

build_url always appended ?since= to the URL, but the Issues and Comments stream paths already contain a query string (e.g. issues?state=all&sort=updated&direction=desc). This produced a malformed URL with a double ?:
/issues?state=all&sort=updated&direction=desc?since=2026-04-30T16:55:02Z

GitHub parses everything after the first ? as the query string, so direction received the value desc?since=... (invalid) and since was never recognized as a parameter. With direction malformed, GitHub could return records in ascending order — causing IncrementalOrderedStream to hit synced_all_records = True on the first old record and drop all recently updated issues.

Manual QA steps

• Ran the tap, updated an Issue and saw that it was not replicated
• Ran new code, verified the updated Issue was replicated.

Risks

Rollback steps

• revert this branch

AI generated code

https://internal.qlik.dev/general/ways-of-working/code-reviews/#guidelines-for-ai-generated-code

• this PR has been written with the help of GitHub Copilot or another generative AI tool