fix(ai-chat): sanitize tool schemas for GitHub Copilot LSP

[datlechin]

Problem

GitHub Copilot tool registration fails with schema validation errors:

Copilot tools registration failed: LSP server error (-32602): Schema validation failed:
- /tools/1/inputSchema/properties/schema/type: Expected string
- /tools/2/inputSchema/properties/max_rows/type: Expected string
- /tools/2/inputSchema/properties/schema/type: Expected string
...

Copilot's LSP schema validator requires every type to be a single string. Our shared ChatToolSchemaBuilder emits type: ["string", "null"] for optional fields (required by OpenAI strict mode), which Copilot rejects.

Provider research

Audited every AI provider for tool schema acceptance:

Provider	type: [X, null]	additionalProperties	Sanitizer?
OpenAI Responses (strict mode)	Required	Required false	None (works as-is)
Anthropic Claude	Accepts	Accepts	None
Gemini function_declarations	Rejected	Rejected	Already in GeminiProvider.sanitizeSchemaForGemini
GitHub Copilot LSP	Rejected	Accepts	This PR
OpenAI-compatible (DeepSeek, OpenRouter, generic)	Accepts	Accepts	None
Ollama	Accepts	Accepts	None

The shared schema builder cannot be changed because OpenAI strict mode requires the current format. Each strict provider sanitizes at its egress point.

Fix

ChatToolSpec+Copilot.swift — replaced normalizeForCopilot (which only added an empty required array) with sanitizeForCopilot:

1. Recursively walks the schema
2. For each property whose type is [X, "null"]:
   • Rewrites to type: X (single string)
   • Strips null from any companion enum array
   • Removes the property from the parent object's required array (so Copilot understands the field is optional via standard JSON Schema semantics)
3. Recurses into nested object schemas and array items

Plain type: "string" properties and required fields are untouched. additionalProperties is preserved (Copilot accepts it).

Tests

6 new tests in CopilotSchemaSanitizationTests:

• rewritesOptionalScalar — [X, null] becomes X, field dropped from required
• preservesRequiredFields — non-nullable scalars stay in required
• stripsNullFromEnum — enum arrays lose null members
• mixedRequiredAndOptional — selective drop, doesn't affect non-nullable siblings
• recursesIntoNested — nested object schemas also sanitized
• realBuilderOutputIsValid — feeds an actual ChatToolSchemaBuilder output through and asserts no type arrays remain and optional fields are excluded from required

All pass on both arm64 and x86_64.

Test plan

• Open AI Chat with GitHub Copilot provider selected
• Send any message that triggers tool registration
• Verify no "Copilot tools registration failed" log
• Verify tools actually fire (list_tables, execute_query, etc.)
• Other providers (Claude, OpenAI, Gemini) still work