feat(client): add platform NemoClient provider

[maxdubrinsky]

Summary

• Add a plugin-side NemoClientProvider protocol with explicit override, entry-point discovery, and standalone environment fallback.
• Add the platform provider backed by service URL routing, shared HTTP transports, request principal headers, and OTEL propagation.
• Register provider entry points in nmp-common and the bundled nemo-platform package.
• Fail closed for broken or conflicting installed providers and prevent OTEL context from replacing authoritative identity headers.

Validation

• 81 focused provider and client-factory tests passed after rebasing onto current main.
• Ruff, formatting, focused ty, wheel entry-point inspection, and git diff --check passed.
• Independent adversarial review found no blocker.

Follow-up

Directly owned NemoClient transport teardown is pre-existing and intentionally outside this PR. AIRCORE-933 tracks ownership-aware close and context-manager support.

Linear: https://linear.app/nvidia/issue/AIRCORE-883

[github-actions]

Suite	Lines Covered	Line Rate	Branch Rate
Unit Tests	26502/34123	77.7%	62.0%
Integration Tests	15214/32748	46.5%	18.7%

[maxdubrinsky]

Folded into #800, which is now retargeted to main and carries the full NemoClient foundation (provider AIRCORE-883 + request-scoped DI AIRCORE-904). Closing to consolidate the two stacked drafts into one reviewable PR. No content lost: #800's branch already contained every commit from this branch.