fix(dashboard): reject MRI queries with actionable tracemetrics guidance (#601)

## Summary

- Detect MRI (Metric Resource Identifier) syntax in `--query` arguments
and reject with a `ValidationError` that suggests the correct `--dataset
tracemetrics` format
- Port of Sentry's canonical Python MRI parser regex from
`sentry/snuba/metrics/naming_layer/mri.py`

## Problem

MRI-style queries like `avg(g:custom/node.runtime.mem.rss@byte)` with
`--dataset metrics` passed CLI validation (since "avg" is a valid
function name) but produced widgets that render as "Internal Error" in
the Sentry dashboard UI.

## Solution

Added `parseMri()` and `rejectMriQueries()` to the aggregate validation
pipeline in `src/types/dashboard.ts`. When MRI syntax is detected, the
error message includes:
- The equivalent tracemetrics query format:
`avg(value,node.runtime.mem.rss,gauge,byte)`
- A prompt to switch to `--dataset tracemetrics` if not already using it

Example error:
```
MRI query syntax is not supported for dashboard widgets: "avg(g:custom/node.runtime.mem.rss@byte)".

Use the tracemetrics query format instead:
  --query 'avg(value,node.runtime.mem.rss,gauge,byte)'
Use --dataset tracemetrics instead of --dataset metrics.

Tracemetrics format: fn(value,<metric_name>,<type>,<unit>)
```

Closes #539

Author: BYK