[pigeon] Reduces lint rules ignored in generated Dart code

[srawlins]

When I landed #11149, which changes the ignore_for_file to use type=lint, I didn't examine the list of lint rules we used to violate. In particular, since #11114 and #11163 landed, our generated code no longer violates the following rules:

• unnecessary_parenthesis
• prefer_null_aware_operators
• unnecessary_import
• no_leading_underscores_for_local_identifiers

I believe this does not need CHANGELOG notes because users should not be using the hidden 'ignore-lints' flag. This list of individually ignored lint rules should only appear in our tests or our checked-in generated Dart files.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[tarrinneal]

If it's good enough for the linter, it's good enough for me @stuartmorgan-g for exceptions