feat(issues): add examples propose subcommand [LSEN-119]

[Palash Shah (Palashio)]

Summary

Adds langsmith project issues examples propose <issue-id> — a CLI command that proposes a run as a regression example for an issue by calling POST /v1/platform/issues/{issue_id}/proposed-examples.

This mirrors the propose_regression_example tool from smith-issues-agent, making the same operation available to users and automation scripts outside the agent context.

Usage:

langsmith project issues examples propose <issue-id> \
  --run-id <run-id> \
  --assertion correctness="Response must cite sources" \
  --assertion format="Output must be valid JSON"

Each --assertion flag takes a key=comment pair. Client-side validation enforces:

• At least one assertion required
• At most 10 assertions
• Unique non-empty keys
• Non-empty comments

The body sent to the API is {"run_id": "...", "assertions": [{"key": "...", "comment": "..."}, ...]}.

Test Plan

• 6 unit tests added in issues_test.go: command structure, flag presence, parseAssertion valid/invalid cases
• go test ./internal/cmd/ passes (full suite)
• go build ./... succeeds

Release Note

none