Add guidance for searching multi-line function call chains in Copilot instructions (Azure#3138)

- [x] Review current copilot-instructions.md file structure
- [x] Add guidance section about searching for function call chains
- [x] Provide examples of multi-line Rust method chaining patterns
- [x] Suggest using ripgrep with multi-line search or AST-based tools
- [x] Condense guidance to reduce token count per feedback
- [x] Add rustfmt to cspell.json word list

## Summary

Updated the Copilot instructions with concise guidance about searching
for multi-line function call chains in Rust code, and added "rustfmt" to
the cspell.json dictionary.

### Changes:
- Removed the separate "Searching for Code Patterns" section to reduce
token count
- Condensed all guidance into a single, concise bullet point in the Code
Generation section
- Removed redundant information while keeping the essential message:
  - Awareness that rustfmt formats method chains across multiple lines
  - Suggestion to use multi-line search patterns (`rg` with `-U` flag)
  - Alternative approach: search for individual method names
- Added "rustfmt" to `.vscode/cspell.json` word list in alphabetical
order

This addresses the issue while being much more token-efficient.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Update Copilot instructions to search for call chains
across multiple lines</issue_title>
> <issue_description>I've noticed lately that Copilot often greps for
function call chains like `obj.foo().bar()` when, in fact, rustfmt
might've reformatted it across multiple lines like,
> 
> ```rust
> obj
>     .foo()
>     .bar();
> ```
> 
> Update the copilot-instructions.md to take this into account whenever
it's grep'ing for function call chains.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes Azure#3137

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: heaths <[email protected]>

Author: Copilot

.github/copilot-instructions.md

@@ -39,6 +39,7 @@ Use these instructions for test generation as well.
 -   If you have trouble generating safe, efficient, maintainable, and lint-free code, insert a `TODO` comment describing what should happen.
 -   All imported types, constants, functions, modules, and macros should be imported explicitly. Never import `*`.
 -   Do not modify generated code, found in `generated` subdirectories. These files are generated by external tools and should not be edited manually.
+-   When searching for function call chains in Rust code, be aware that rustfmt often formats method chains across multiple lines like `obj\n    .foo()\n    .bar()`. Use multi-line search patterns (e.g., `rg` with `-U` flag) or search for individual method names rather than complete call chains.
 
 ## Test Generation
 

.vscode/cspell.json

@@ -74,6 +74,7 @@
     "rsplit",
     "rustfmt",
     "runtimes",
+    "rustfmt",
     "rustup",
     "schannel",
     "seekable",