Stop using html/template in U2M OAuth callback to restore linker DCE (fixes #343)

[vikrantpuppala]

Summary

Reachable use of the stdlib html/template in package auth/oauth/u2m disabled the Go linker's public dead-code elimination (DCE) for the entire binary of any application that imports databricks-sql-go. html/template builds on text/template, whose reflection-based field evaluation forces the linker to retain code it would otherwise strip. This bloats consumer binaries — up to ~20% (40 MB) in multi-library binaries like xo/usql, as reported in #343.

This PR rewrites the mostly-static OAuth callback page as plain string building with explicit html.EscapeString on the dynamic fields (the same approach xo/tblfmt used for a similar issue), removing html/template/text/template from the reachable graph. The now-unused embedded templates/simple.html is deleted.

The dynamic fields remain HTML-escaped, so the escaping behavior of the previous html/template implementation is preserved.

Impact

Measured with a minimal main.go that imports the driver (the repro from the issue):

	Before	After
text/template/html/template symbols (go tool nm)	615	0
Binary size	23.7 MB	14.4 MB (~39% smaller)

The size delta is larger than the driver's own footprint because re-enabling DCE benefits the whole binary, including other libraries.

Test plan

• Added auth/oauth/u2m/html_template_test.go:
  • TestRenderHTML (table-driven): basic field rendering + escaping of <, >, &, "; the conditional action-link block; the conditional code block; and that each page is a complete, well-formed document.
  • TestInfoHTML: success page title/heading/content, no code block.
  • TestErrorHTML: error page title and that the error message is HTML-escaped (no raw <script> leaks through).
• go build ./..., go vet ./auth/oauth/u2m/, and gofmt -l are all clean.
• go test ./auth/oauth/u2m/ passes.
• Verified via go tool nm on a consumer binary that the template symbols are gone (615 → 0) and the binary shrinks as shown above.

Notes

• No other html/template or text/template usages exist in the codebase — this was the sole occurrence of the pattern.
• CHANGELOG updated under an Unreleased section.

Closes #343

This pull request and its description were written by Isaac.