fix: surface MCP initialize errors in UI

[claytonsheets7]

Improves Inspector V1 connection error UX when an MCP server returns a JSON-RPC/MCP error during initialize (notably -32602
Unsupported protocol version). Instead of showing generic “proxy session token” guidance, the Sidebar now surfaces the actual MCP
error code and message, with a small expandable “Error details” section and a spec link for version negotiation.

This makes protocol version failures immediately visible and avoids misleading guidance when the error is not proxy-related.

Note: Inspector V2 is under development to address architectural and UX improvements. During this time, V1 contributions should
focus on bug fixes and MCP spec compliance. See CONTRIBUTING.md (../CONTRIBUTING.md) for more details.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (no functional changes)
• Test updates
• Build/CI improvements

Changes Made

• Sidebar now detects MCP/JSON-RPC errors and renders MCP error : in the connection status area.
• Adds an expandable error details block (data-testid="connection-error-details") and, for -32602, a neutral note with a link to
  the spec’s version negotiation section. Supported versions are shown when present in the error message.
• Proxy token hint now only appears for connectionType === "proxy" and is fully suppressed when an MCP error is present.

Related Issues

• #962
• #959

Testing

• Tested in UI mode
• Tested in CLI mode
• Tested with STDIO transport
• Tested with SSE transport
• Tested with Streamable HTTP transport
• Added/updated automated tests
• Manual testing performed

Automated

• npm test --workspace=client
• npx playwright test -c client/playwright.config.ts client/e2e/protocol-version-error.spec.ts

Repro validation (confidence check)

• On origin/main, the fixed-behavior e2e assertions fail because MCP error details are not surfaced.
• When assertions are temporarily inverted to check the legacy behavior (proxy token hint visible, no error details), the test
  passes on origin/main.
• On this PR branch, the fixed-behavior e2e test passes, confirming the change resolves the issue.

Checklist

• Code follows the style guidelines (ran npm run prettier-fix)
• Self-review completed
• Code is commented where necessary
• Documentation updated (README, comments, etc.)

Breaking Changes

None.

Additional Context

• Formatting: npm run prettier-check passes (no changes required).
• Note: OpenAI Codex was used to assist with drafting parts of this change. All behavior and tests were reviewed and validated locally.