docs: sentence case headings and small accuracy fixes

[mintlify]

Summary

Documentation cleanup pass enforcing the project style guide and correcting a few small inaccuracies.

Style fixes (sentence case for all headings)

Per docs/AGENTS.md, headings must use sentence case. Updated headings across:

• concepts/providers.mdx, concepts/models.mdx, concepts/messages.mdx, concepts/configuration.mdx, concepts/error-handling.mdx
• api/core/errors.mdx, api/core/embed.mdx, api/core/define-tool.mdx, api/core/generate-image.mdx
• All guides (chat-completion, streaming, structured-outputs, tool-calling, embeddings, image-generation, multi-modal)
• examples/overview.mdx

Updated <Card> titles within those pages to match.

Accuracy fixes

• api/core/errors.mdx — name was incorrectly documented as always 'CoreAIError'. Each subclass overrides name; updated the description.
• api/core/define-tool.mdx — Two examples used Zod 3 z.record(z.unknown()) syntax. The library requires Zod 4 (per installation.mdx), which uses z.record(z.string(), z.unknown()).
• installation.mdx — OpenAI install Note now lists tool calling, vision, and reasoning to match the other provider Notes.
• concepts/providers.mdx — Removed the misleading "// Optional: defaults to OPENAI_API_KEY env var" inline comment on a snippet that already passes process.env.OPENAI_API_KEY explicitly. Replaced with a short clarifying sentence.

Validation

• mint validate passes
• mint broken-links reports no broken links