Test coverage for mod_sed y/// size mismatch handling (#678)

[notroj]

Analysis by @claude:

Adds test/modules/filters/test_002_sed_ycomp.py covering the fix in #678, together with that PR's commit so the new cases have something to run against. It extends the existing test/modules/filters package rather than adding one; env.py gains mod_sed.

Five cases: three y/// expressions whose source is longer than the destination, each of which must be rejected naming the size mismatch and not a missing delimiter; one expression whose destination is longer, which the check after the loop already catches correctly today; and one well-formed y/// that must still compile and start the server. Without #678 the first three fail — the parser keeps advancing past the closing delimiter and past the NUL terminating the config line, so the error finally reported is ending delimiter missing on string: y/abcdef/de/ rather than transform strings not the same size. That misreported error is what makes the out-of-bounds read observable from a test without ASan. With #678 applied, all 21 cases in the package pass.

One deliberate change from #678 as submitted: its CHANGES entry is moved to changes-entries/sed-ycomp-mismatch.txt per README.CHANGES, which also resolves a conflict against the current top of CHANGES. modules/filters/sed0.c is untouched and identical to the PR.

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