feat(provider): support OpenAI backend models

[rich7420]

Summary

• Add provider/model compatibility normalization so OpenAI, openai-compatible, Anthropic, and Vertex paths keep compatible main and heartbeat models.
• Split local config/env credential resolution into a ProviderAccess boundary that can later accept credentials from an API gateway broker without replacing Claude support.
• Send GPT-5-family requests with max_completion_tokens, keep legacy GPT-4o-style models on max_tokens, and apply model-specific output-token caps.
• Update gateway config/session reload paths so provider/key/model changes validate before persisting and live sessions/heartbeat pick up compatible config.

Validation

• bun run typecheck
• bun run test: 4706 pass, 5 skip, 0 fail
• bun test --timeout 30000 --max-concurrency=1 ./tests/e2e-gateway.ts: 20 pass, 0 fail
• bun test --timeout 30000 --max-concurrency=1 ./tests/e2e-gateway-sessions.ts ./tests/e2e-bridge-reply.ts ./tests/e2e-chart-pipeline.ts ./tests/e2e-generate-chart.ts: 21 pass, 0 fail
• bun test --timeout 60000 --max-concurrency=1 ./tests/e2e-memory-distill.ts ./tests/e2e-person-rpc.ts ./tests/e2e-send-photo.ts ./tests/e2e-web-demo.ts ./tests/e2e-where-reminder.ts: 26 pass, 0 fail
• ANTHROPIC_API_KEY from local config + bun test --timeout 60000 --max-concurrency=1 ./tests/e2e-cron.ts ./tests/e2e-heartbeat.ts: 7 pass, 0 fail
• Live gateway smoke against http://127.0.0.1:4242: /health, /auth/device, WebSocket connect, config.get, provider.listModels, and chat.send all passed; second chat returned SMOKE_OK.
• Real OpenAIProvider.stream smoke: gpt-5.5 and gpt-4o-mini both returned complete streaming events.
• Real OpenAIProvider tool-call smoke: gpt-5.5 produced probe_echo tool_use, accepted tool_result, and returned probe-result:alpha after the tool result.

Limitations / Follow-ups

1. This PR only adds local OpenAI key support for backend provider execution; the future API gateway credential broker can plug into the ProviderAccess boundary later.
2. Existing Claude/Anthropic support is preserved. When switching providers, stale incompatible heartbeat overrides are normalized instead of reused.
3. Full ./tests/e2e-*.ts cannot be run completely in this local shell because BRAVE_API_KEY is not configured and direct AnthropicProvider smoke currently returns connection_error in this environment; cron/heartbeat Anthropic-backed e2e still pass when ANTHROPIC_API_KEY is injected from local config.