fix: prevent panic when BaseURL has empty scheme in normalizeRemoteURL

[toller892]

Description

normalizeRemoteURL unconditionally overwrites the remote URL's scheme with the BaseURL's scheme, even when BaseURL has an empty scheme. This clears the remote URL's scheme, leading to a (nil, nil) return at the empty-scheme check in OpenWithContext (line 588-590). Callers like consumeAdaptedFile then pass nil to io.ReadAll, causing an unrecoverable panic.

In some call chains this happens from a singleflight.Group.Do goroutine, making it impossible for the calling program to recover even with recover.

Changes

1. normalizeRemoteURL: Skip normalization entirely when BaseURL lacks a host or scheme. This prevents the remote URL's scheme from being cleared by a malformed base URL.

2. OpenWithContext: Return an error instead of (nil, nil) when the scheme is empty after normalization (defense-in-depth). This prevents io.ReadAll(nil) panics even if some other code path produces an empty scheme.

3. Regression tests: Added TestNormalizeRemoteURL_PreservesSchemeWhenBaseURLHasEmptyScheme, TestNormalizeRemoteURL_OverwritesSchemeWhenBaseURLHasScheme, TestNormalizeRemoteURL_NoOpWhenBaseURLIsEmpty, and TestOpenWithContext_ReturnsErrorOnEmptyScheme.

Testing

All existing tests pass, including TestNewRemoteFS_BasicCheck_NoScheme (updated to expect an error instead of nil when the root URL is empty, since normalization is now a no-op and the URL is fetched normally).

ok  github.com/pb33f/libopenapi/index   71.390s

Fixes #578

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.71%. Comparing base (5957faa) to head (3698b1b).

Files with missing lines	Patch %	Lines
index/rolodex_remote_loader.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
- Coverage   99.72%   99.71%   -0.01%     
==========================================
  Files         280      280              
  Lines       33853    33855       +2     
==========================================
- Hits        33761    33760       -1     
- Misses         55       57       +2     
- Partials       37       38       +1     

Flag	Coverage Δ
unittests	99.71% <66.66%> (-0.01%)	⬇️

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[daveshanley]

One single line.. you can do it!