[flutter_tools] Compute LSP Content-Length header using UTF-8 byte length (#191377)

## Description

Fixes `flutter analyze` crashing with `FormatException: Unexpected end
of input` when analyzing a project whose path contains non-ASCII
characters (e.g. `Alexander’s MacBook Pro` with right single quotation
mark `’` / U+2019).

In `AnalysisServer._writeMessage`, the `Content-Length` header was being
calculated using `message.length` (UTF-16 code units) rather than the
UTF-8 encoded byte length required by the LSP specification. When
messages contained multibyte Unicode characters (such as in `rootUri` or
`workspaceFolders`), the header under-reported the payload length,
causing the analysis server's JSON parser to receive a truncated payload
and exit prematurely with code 255.

Also updates `MockLspServerProcess` to parse stdin as a stream of raw
bytes and compute outgoing message length in bytes to faithfully
simulate the LSP transport layer.

## Related Issues
Fixes https://github.com/flutter/flutter/issues/191309

## Tests
- Added `AnalysisServer handles non-ASCII project path` unit test in
`packages/flutter_tools/test/commands.shard/hermetic/analyze_continuously_test.dart`.

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responsibilities.
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we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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