perf(cli): read a single on-chain issue by ID instead of scanning all

[kinjitakabe]

Why this is necessary: fetch_issue_from_contract is used to act on one issue by ID (e.g. gitt issues submissions --id <N>), but it currently reads every issue registered on the contract to find it:

issues = read_issues_from_contract(...)          # one childstate RPC per issue, 1..next_issue_id
issue = next((i for i in issues if i.get('id') == issue_id), None)

That's O(N) substrate RPC round-trips to fetch O(1) data. next_issue_id only ever grows — completed and cancelled issues are never removed from storage — so every single-issue command gets progressively slower and pounds the node with N round-trips for data whose storage key it can compute directly. The Ink! 5 lazy mapping key for a specific issue_id is already computable inline, so a single read was always possible; the code just wasn't using it.

The fix: add read_issue_from_contract, which computes the one issue's lazy key and does a single childstate_getStorage call, and route fetch_issue_from_contract through it. O(N) → O(1).

The per-issue decode is factored into a shared _read_one_issue_from_child_storage helper, so the full-scan path (read_issues_from_contract, used by issues list) and the new single-read path share one decode implementation instead of duplicating it. Behavior is unchanged: fetch_issue_from_contract still raises the same ClickExceptions for not-found, non-bountied, and incomplete issues.

Related Issues

N/A

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (performance)

Testing

• New tests/cli/test_issue_single_read.py: single read does exactly one RPC,
  returns None when absent, fetch_issue_from_contract uses the single read
  (asserts the full scan is not called), and the not-found / non-bountied
  paths still raise ClickException.
• uv run pytest — 882 passed.
• uv run pyright / ruff check / vulture on changed files — clean.

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[anderdc]

fix conflicts

[anderdc]

Fix conflicts. Heads up: test now distinguishes a contract read error (ClickException "Error reading from contract: …") from a genuine not-found in fetch_issue_from_contract. read_issue_from_contract here swallows exceptions and returns None, which would report connection failures as "Issue ID N not found on-chain." Preserve that distinction when you rebase.