fix: 4 additional raise sites missing exception chaining with 'from'

[siddhirajkatkar]

Follow-up to #2564 which fixed 12 sites. After reviewing the codebase,
I found 4 additional sites that were missed.

Sites

File	Line	Fix
server/mcpserver/resources/templates.py	133	add from e
shared/experimental/tasks/in_memory_task_store.py	172	add from None
server/mcpserver/prompts/base.py	184	add from None
server/mcpserver/prompts/base.py	189	add from e

Impact

Same as #2564 — without from, Python shows
"During handling of the above exception, another exception occurred"
instead of "The above exception was the direct cause".
Callers cannot inspect __cause__ programmatically.

I have a PR ready for this.