Show line range in agent host file-read tool display (#312062)

* Show line range in agent host file-read tool display

When the Copilot CLI's `view` tool is called with a `view_range`, surface
the line range in the invocation and past-tense messages so users see e.g.
"Reading file.ts, lines 10 to 20" instead of just "Reading file.ts".

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Match Copilot Chat extension's view_range validation

Drop the EOF-sentinel handling and require a strictly valid two-element
range (`length === 2`, integers, `start >= 0`, `end >= start`); fall back
to the path-only display otherwise. Mirrors `formatViewToolInvocation`
in the Copilot Chat extension and addresses review feedback on the
`-1` end-of-file sentinel and the stale doc comment.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Handle view_range `-1` end-of-file sentinel

The Copilot CLI uses `-1` as the documented "to end of file" sentinel
for the second element of `view_range`. The Copilot Chat extension
doesn't handle this and either drops the range or renders "-1"
literally; do better here by rendering "from line {n} to the end".

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Tweak EOF view_range wording

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>