docs: correct the Python and Go event data model tip

[examon]

What

docs/features/streaming-events.md told Python and Go users that session event payloads use a single Data class/struct with every field optional, and that fields not populated for an event come back as None/nil. Both bindings use a separate data type per event, so neither behaves that way.

Fixes #2159

Why the old text was wrong

Go. event.Data is the SessionEventData interface (go/rpc/zsession_events.go), which carries no data fields, so reading one off it does not compile:

./main.go:41:35: event.Data.ToolName undefined (type rpc.SessionEventData has no field or method ToolName)

toolName is listed in the doc's own tool.execution_start table, so this is a field the tip said was readable for that event.

Python. event.data is a per-event dataclass, so reading a field that belongs to a different event raises instead of returning None:

AttributeError: 'ToolExecutionStartData' object has no attribute 'delta_content'

The class named Data still exists in the Python binding, but it is a shim for manually constructed payloads with no declared fields; event delivery never produces it, and unrecognized events fall back to RawSessionEventData.

The tip was accurate when it was written. Go moved to per-event data structs in #1037 and Python in #1063. The Go change updated this file's Go example and left the prose behind. Docs validation extracts and compiles fenced code blocks only, so nothing checks prose, which is why this file's own Go example uses event.Data.(*copilot.AssistantMessageDeltaData) about sixty lines above a tip saying that is unnecessary.

What changed

• docs/features/streaming-events.md - the Python/Go tip now describes per-event data types, using the wording the file already uses for the .NET tip directly below it.
• .github/instructions/docs-style.instructions.md - the same sentence is used there as the example of a language-qualified callout, so it carried the same incorrect claim.

Prose only. No code, generated files, codegen templates, schemas, samples or tests.

Verification

• Programs written to follow the corrected tip compile and run in both Go and Python against a build of this branch. Programs written to follow the old tip still fail with the errors above.
• npm run validate:go (48/48) and npm run validate:py (57/57) pass, and the extracted code corpus is byte-identical to main since only prose changed.
• The corrected tip renders as a Tip callout.

Not included

Java uses per-event classes and Rust exposes the payload as untyped JSON. Neither has a tip in this section today and this change does not add one.

[Copilot]

Pull request overview

Corrects Python and Go streaming-event documentation to reflect their per-event payload types.

Changes:

• Updates the streaming-events tip.
• Aligns the docs style-guide example.

Show a summary per file

File	Description
docs/features/streaming-events.md	Corrects the Python and Go event data model guidance.
.github/instructions/docs-style.instructions.md	Updates the language-qualified callout example.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Medium