Reader.peekDelimiterInclusive: Fix handling of `stream` implementations that return 0

Previously, the logic in peekDelimiterInclusive (when the delimiter was not found in the existing buffer) used the `n` returned from `r.vtable.stream` as the length of the slice to check, but it's valid for `vtable.stream` implementations to return 0 if they wrote to the buffer instead of `w`. In that scenario, the `indexOfScalarPos` would be given a 0-length slice so it would never be able to find the delimiter.

This commit changes the logic to assume that `r.vtable.stream` can both:
- return 0, and
- modify seek/end (i.e. it's also valid for a `vtable.stream` implementation to rebase)

Also introduces `std.testing.ReaderIndirect` which helps in being able to test against Reader implementations that return 0 from `stream`/`readVec`

Fixes #25428

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