perf(issue-discovery): pool HTTP connections across scan_closed_issues round

[bitloi]

Summary

execute_graphql_query in gittensor/utils/github_api_tools.py uses top-level requests.post calls, so every invocation pays a fresh TCP + TLS handshake to api.github.com. This affects all GraphQL callers, including find_solver_from_cross_references → _search_issue_referencing_prs_graphql → execute_graphql_query.

Proposed improvement:

• Add an optional session: Optional[requests.Session] = None parameter to execute_graphql_query, _search_issue_referencing_prs_graphql, and find_solver_from_cross_references.
• When a session is provided, use session.post(...) instead of requests.post(...) so urllib3 reuses the underlying connection pool.
• Default None preserves all existing callers and tests unchanged; this is a transport-layer optimization only.

Motivation

Any hot path that issues multiple GraphQL calls per round (issue discovery, solver lookups, etc.) benefits from connection reuse. Session pooling would:

• Reduce repeated TCP/TLS setup overhead per call.
• Improve wall-clock latency stability under load.
• Lower avoidable CPU/network overhead without changing scoring logic.

This is complementary to #687, which applies the same pooling idea to the OSS miner-scoring path.