docs(protocol): define invalid base64, late output and retention for v2 terminals (#2548)

Display-only terminals didn't say what to do with data that isn't valid
base64, output after a concrete exitStatus, or how much output to keep.
Clients ignore invalid chunks or snapshots and keep the bytes they have,
still append output that arrives after exit (Agents should send it
before), and may bound retained output, with a later snapshot replacing
it.

Author: benbrandt

docs/protocol/v2/draft/tool-calls.mdx

@@ -501,6 +501,12 @@ sequences, so decoders retain parser state across chunks. Optional nullable
 `_meta` is chunk-scoped; omission and `null` both mean no chunk metadata was
 provided.
 
+Clients **SHOULD** ignore a chunk or snapshot whose `data` isn't valid base64,
+keeping the bytes they already have. Agents **SHOULD** send all output before a
+concrete `exitStatus`, but Clients still append output that arrives after it.
+Clients **MAY** bound how much output they keep, for example by dropping the
+oldest bytes; a later `output` snapshot replaces whatever they kept.
+
 Terminal output is Agent content. Clients **SHOULD** render it so that it cannot write the clipboard, send input or query responses, or open links without user action, and sanitizers **SHOULD** keep state across chunks.
 
 ### Diffs

docs/protocol/v2/tool-calls.mdx

@@ -501,6 +501,12 @@ sequences, so decoders retain parser state across chunks. Optional nullable
 `_meta` is chunk-scoped; omission and `null` both mean no chunk metadata was
 provided.
 
+Clients **SHOULD** ignore a chunk or snapshot whose `data` isn't valid base64,
+keeping the bytes they already have. Agents **SHOULD** send all output before a
+concrete `exitStatus`, but Clients still append output that arrives after it.
+Clients **MAY** bound how much output they keep, for example by dropping the
+oldest bytes; a later `output` snapshot replaces whatever they kept.
+
 Terminal output is Agent content. Clients **SHOULD** render it so that it cannot write the clipboard, send input or query responses, or open links without user action, and sanitizers **SHOULD** keep state across chunks.
 
 ### Diffs