fix(client): improve request failure diagnostics

[Vizards]

Summary

• Add request context to DeepSeek HTTP and network failure diagnostics, including base URL, model, request shape, and response details.
• Keep Output logs readable by using a compact first-line diagnostic summary with the raw stack below it.
• Preserve nested network cause details as a single-line JSON payload without dumping the whole error object as formatted output.

Validation

• npm run compile
• npm run format:check
• npm run lint
• git diff --check

[Copilot]

Pull request overview

This PR improves DeepSeek client failure diagnostics by attaching request context (base URL + request shape metadata) to HTTP/network errors and emitting a compact, readable first-line summary with the raw stack beneath it.

Changes:

• Introduces a RequestErrorContext type to carry base URL + request details into error handling.
• Enhances HTTP and network error diagnostic strings to include request metadata (model, stream, tool/message counts, etc.) and safer single-line serialization.
• Updates client logging to emit a single formatted diagnostic+stack string via formatRequestError.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/client/types.ts	Adds RequestErrorContext to pass request/baseUrl context into error formatting.
src/client/error.ts	Extends diagnostic formatting for HTTP/network/unknown errors and adds formatRequestError.
src/client/core.ts	Passes request context into error creation/normalization and updates logging to use the new formatter.

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