Improve diagnostics for the case of missing tag name in `#enddocfragment`

[alexander-yevsyukov]

I forgot to put a tag name after #enddocfragment and got this:

> Task :docs:embedCode FAILED
Using user's local bash profile.
export plugin already enabled
2026/02/16 17:51:17 INFO starting application, reading args...
panic: runtime error: slice bounds out of range [23:22]

goroutine 1 [running]:
embed-code/embed-code-go/fragmentation.lookup({0x14000d25b60, 0x16}, {0x100fad0c3, 0xf})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/lookup.go:71 +0x1b0
embed-code/embed-code-go/fragmentation.FindEndDocFragments(...)
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/lookup.go:53
embed-code/embed-code-go/fragmentation.Fragmentation.parseLine({{{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...}, ...}, ...)
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/fragmentation.go:242 +0x70
embed-code/embed-code-go/fragmentation.Fragmentation.DoFragmentation({{{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/fragmentation.go:115 +0x14c
embed-code/embed-code-go/fragmentation.Fragmentation.WriteFragments({{{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/fragmentation.go:137 +0x40
embed-code/embed-code-go/fragmentation.writeFragments({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...}, ...)
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/fragmentation.go:204 +0x9c
embed-code/embed-code-go/fragmentation.WriteFragmentFiles({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/fragmentation/fragmentation.go:177 +0x15c
embed-code/embed-code-go/cli.EmbedCodeSamples({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10111e6c0, 0x1, 0x1}, {0x10111fc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/cli/cli.go:107 +0x44
main.main()
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/main.go:111 +0x48c

[Incubating] Problems report is available at: file:///Users/sanders/Projects/Spine/validation/build/reports/problems/problems-report.html

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':docs:embedCode'.
> Process 'command '/Users/sanders/Projects/Spine/validation/docs/_script/embed-code'' finished with non-zero exit value 2

It took some time to figure out the cause.

It's natural to assume that the tag name is not needed because it resembles closing braces.
Yes, of course, there is a case of overlapping fragments and it is why we require the name of the fragment in the closing tag.

It would be helpful to have a meaningful error message which points out the absence of the name of the tag.