bug: legacy `base_url` configs are not honored consistently

Problem

The extension documentation says legacy base_url is still supported as a generic alias for provider endpoints, but the runtime only uses provider-specific base URL keys in several critical paths.

Observed behavior

Request path ignores generic base_url

• BaseHttpProvider.sendChatRequest() resolves the endpoint from baseUrlConfigKey only.
• OpenAIProvider, ClaudeProvider, and GeminiProvider therefore ignore base_url= unless a provider-specific key is also present.

Ollama readiness and request paths disagree

• LLMExtension.isOllamaReachable() resolves ollama_base_url or base_url, but then writes the resolved value back to ConfigStore.BASE_URL.
• OllamaProvider.checkServerConnection() and OllamaProvider.listInstalledModels() only read ollama_base_url.
• This means the readiness check, provider-status output, and actual request path can disagree depending on which key the user set.

Why this matters

The docs and sample configs currently encourage base_url in multiple places. Users can follow the documented config format and still hit the wrong endpoint, especially with Ollama or custom/OpenAI-compatible deployments.

This is more than a documentation mismatch:

• a config can validate against one URL and send requests to another
• provider-status can report a different Ollama target than the actual provider uses
• legacy configs that should remain backward-compatible are effectively broken

Affected code

• src/main/providers/BaseHttpProvider.scala
• src/main/providers/OllamaProvider.scala
• src/main/LLMExtension.scala
• docs/CONFIGURATION.md
• docs/USAGE.md
• demos/config

Suggested fix directions

1. Normalize generic keys to provider-specific keys when loading config.
2. Or make every provider explicitly fall back from provider-specific URL to generic base_url.
3. Use the same resolution path for readiness checks, provider-status, and actual request dispatch.
4. Update docs/examples to match whichever behavior is kept.

Start by tracing URL resolution in src/main/providers/BaseHttpProvider.scala, src/main/providers/OllamaProvider.scala, and src/main/LLMExtension.scala, then compare the affected examples in docs/CONFIGURATION.md, docs/USAGE.md, and demos/config. Establish one resolution path for generic and provider-specific keys across readiness, status, and requests, and update the documentation and examples so they describe the resulting behavior.