Require '#tool:' prefix for tool references in prompt/instruction/agent files (#272565)

* Require '#tool:' prefix for tool references in prompt/instruction/agent files

* Fix tests

* Don't match parentheses or brackets

* Stricter parsing for tool names

* Small comment fix

* Improve regex match group names and comments

* Fix semantic tokens provider

* support tools with `/` and `.` in qualified names

* improve completions, add tests

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Co-authored-by: Martin Aeschlimann <[email protected]>