fix: send coder User-Agent on all deployment requests

[untra]

• DEVEX-778

Summary

.NET's HttpClient omits User-Agent by default, causing AWS WAF to reject requests with 403 on deployments behind an ALB. This blocked sign-in (/api/v2/buildinfo, /api/v2/users/me), and — because Desktop downloads the tunnel binary from the deployment rather than bundling it — Coder Connect could not start either. Closes #176.

What changed

• CoderSdk/UserAgent.cs (new): formats User-Agent as <token>/<version> (<goos>/<goarch>),
  e.g. coder-desktop/0.8.4 (windows/amd64). Platform names follow Go's GOOS/GOARCH rather than
  .NET's spelling, so a single pattern matches the CLI, the vpn-daemon and every Desktop client.

• SDK HTTP clients: JsonHttpClient — the single chokepoint both CoderApiClient and
  AgentApiClient route through — now sets DefaultRequestHeaders.UserAgent, selected by a
  CoderComponent enum (Desktop / Core). DefaultRequestHeaders is deliberate: .NET applies it
  only when the request doesn't already carry the header, so caller-supplied headers still win.

• Vpn.Service: Manager.cs and Downloader.cs identify as CoderComponent.Core, covering the
  buildinfo/users-me calls and the tunnel binary download.

Validation

• Tests.CoderSdk: 8 → 14 passing (+6 UserAgentTest).
• Tests.Vpn.Service/DownloaderTest.cs: 26 → 28 (+WithUserAgent, +WithUserAg Both assert the header on the wire via the existing TestHttpServerfixture, including that a caller-suppliedUser-Agent` overrides the default.
• Tested build in parallels: dotnet build clean, built and ran the arm build with changes, run in windows parallels

[linear-code]

DEVEX-778