docs: add troubleshooting guide for silent model fallback

[patniko]

Summary

Adds a new "Silent Model Fallback / Wrong Model Being Used" section to docs/troubleshooting/debugging.md under Common Issues.

Problem

When users specify an invalid or unavailable model ID in their session configuration, the Copilot CLI silently falls back to a default model (e.g. claude-sonnet-4.6) instead of raising an error. This is confusing because the SDK accepts any string as a model ID without client-side validation.

What's Added

A troubleshooting entry that documents:

1. Symptoms and root cause of the silent fallback behavior
2. Workaround: Pre-validate — call listModels() before session creation to verify the model exists
3. Workaround: Detect at runtime — compare assistant.usage event model against the requested model
4. Workaround: Query active model — use rpc.model.getCurrent() post-creation to confirm
5. Workaround: Debug logging — enable verbose logging to surface server-side fallback warnings

All code examples cover the four SDKs (Node.js, Python, Go, .NET) using the existing <details> tab pattern.

Relates to #250

[Copilot]

Pull request overview

Adds troubleshooting documentation for cases where specifying an invalid/unavailable model ID results in the session using a different model without an SDK error.

Changes:

• Adds a new “Silent Model Fallback / Wrong Model Being Used” entry under Common Issues.
• Documents symptoms/root cause and provides multiple workarounds (pre-validate, runtime detection, query active model, debug logging).

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[Copilot]

The PR description says “All code examples cover the four SDKs (Node.js, Python, Go, .NET)”, but the “Detect fallback at runtime” workaround only includes Node.js and Python. Add Go and .NET examples in the same <details> pattern, or update the PR description/section text to reflect the actual coverage.

[Copilot]

Listing a specific fallback order is likely to become outdated and can mislead readers if the server behavior changes. Consider rephrasing to a more stable statement (e.g., “falls back to a default available model (often …); exact selection may vary by account/availability”) and avoid implying a guaranteed ordering.

Suggested change

	it silently falls back to the first available default model rather than returning an error.
	The fallback order is `claude-sonnet-4.6` → `claude-sonnet-4.5` → `claude-haiku-4.5` → etc.
	Warnings are logged server-side but are not surfaced to the SDK as errors.
	it silently falls back to a default available model rather than returning an error. This is often
	a `claude-sonnet` or `claude-haiku` variant, but the exact model selected may vary by account,
	configuration, and model availability. Warnings are logged server-side but are not surfaced to
	the SDK as errors.

[Copilot]

The prose says to “use rpc.model.getCurrent()”, but the Go and .NET sections use different-looking APIs (session.Model.GetCurrent / session.GetModelAsync). To avoid confusion, either (a) adjust the text to “use the SDK-specific equivalent of the model get-current RPC” or (b) align the examples/method naming consistently with the same conceptual API across languages.

Suggested change

	3. **Query the active model** after session creation to confirm what model is in use:
	3. **Query the active model** after session creation to confirm what model is in use. For each language, use the SDK-specific equivalent of the model get-current RPC:

[Copilot]

This Go example is presented as a snippet, but it relies on fmt/strings without showing imports, and the indentation is not gofmt-style (spaces instead of tabs inside the for and if). Consider either showing a complete, gofmt’d snippet (with imports) or adding a brief note about required imports and formatting to keep the example copy/paste-friendly.

Suggested change

	models, err := client.ListModels(ctx)
	desiredModel := "gpt-5.4"
	found := false
	var ids []string
	for _, m := range models {
	ids = append(ids, m.ID)
	if m.ID == desiredModel {
	found = true
	}
	}
	if !found {
	panic(fmt.Sprintf("Model %q is not available. Available: %s",
	desiredModel, strings.Join(ids, ", ")))
	// Requires imports "fmt" and "strings".
	models, err := client.ListModels(ctx)
	desiredModel := "gpt-5.4"
	found := false
	var ids []string
	for _, m := range models {
	ids = append(ids, m.ID)
	if m.ID == desiredModel {
	found = true
	}
	}
	if !found {
	panic(fmt.Sprintf("Model %q is not available. Available: %s",
	desiredModel, strings.Join(ids, ", ")))