voice: the speech shaper drops ``` fences but still reads ~~~ fences and indented code blocks aloud #17953

Found in review of the #13102 speech shaper (createSpeechShaper in autobot-frontend/src/utils/ttsSentences.ts).

The shaper drops fenced code blocks delimited by ``` (including a marker split across streamed slices), but two other markdown code-block forms are still read aloud:

• ~~~ fences. The tildes are removed by the emphasis rule, and the code body is spoken.
• Indented code blocks (4-space or tab indented lines after a blank line).

Both are rarer than backtick fences in assistant output, but when they appear the same "symbol soup" problem as #13102 returns.

Acceptance criteria

• ~~~ fences are dropped like ``` fences, including when the marker splits across two streamed slices. The partial-marker hold must not swallow ~~strikethrough~~.
• Indented code blocks are dropped from the spoken copy, without dropping indented prose such as nested list items.
• Tests cover both forms and the prose cases that must survive.