stripGeneratedImageEchoes stripped EVERY markdown image and #media: link
from a final message whenever an image_generate tool result was present
in the same turn's parts. A completed response that carried a MEDIA:
directive for a different file lost its attachment link entirely, and a
message whose only content was such a directive rendered blank after the
empty text part was dropped by dedupeGeneratedImageEchoesInParts
(#80621).
Scope the strip to echoes of the actual generated source:
- markdown images are removed only when the src matches a source,
with a lookahead so a longer path (a.png.bak) is not eaten by a
prefix source (a.png)
- #media: links are removed only when the encoded path matches a
source, anchored to the closing paren so prefix paths survive
- bare-path stripping no longer matches inside markdown link
destinations, so [label](/tmp/a.png) is not corrupted to [label]()
Also wrap parseMarkdownIntoBlocksCached in a lexer-throw fallback so a
single pathological message cannot unwind through MessageRenderBoundary
to the root boundary and blank the whole transcript.
Closes#80621
Rebased onto current main: the strip keeps main's whitespace semantics — main deliberately stopped normalizing whole-document whitespace through media extraction (3e1b7af21b), so the same-file echo test now expects the two-space gap the removed image leaves instead of the old collapsed form. The scoped strip itself is unchanged.
aed18fdfbf