test(sag): cover api client requests

[gregkonush]

Summary

• Adds SAG API client coverage for AgentRun database-access request paths with URL encoding.
• Verifies natural-language task submissions are sent as JSON POST requests.
• Covers failed JSON response handling so server policy errors are surfaced to callers.

Related Issues

None

Testing

• PASS: bun install --frozen-lockfile
• PASS: bun run --filter @proompteng/sag test -- src/lib/sag-client.test.ts
• PASS: bun run --filter @proompteng/sag test
• PASS: bun run --filter @proompteng/sag tsc
• PASS: bun run --filter @proompteng/sag lint
• PASS: bun run --filter @proompteng/sag lint:oxlint
• PASS: bun run --filter @proompteng/sag build

Screenshots (if applicable)

N/A

Breaking Changes

None

Checklist

• Testing section documents the exact validation performed (or N/A with justification).
• Screenshots and Breaking Changes sections are handled appropriately (removed or filled in).
• Documentation, release notes, and follow-ups are updated or tracked.

[gregkonush]

Status: complete. PR is open, mergeable, and CI monitoring is complete.

• Added services/sag/src/lib/sag-client.test.ts for SAG API client request behavior.
• Local validation is green: install, targeted test, full SAG test, typecheck, formatter, Oxlint, and build.
• GitHub reports PR test(sag): cover api client requests #6590 as MERGEABLE / CLEAN; no required checks are configured on the branch.
• PR: test(sag): cover api client requests #6590