Fix file annotation content reuse in watch mode

[sorinc03]

Fixes #736.

Summary

• Clear per-file annotated content at the start of each generation pass.
• Add a regression test covering repeated generation with the same Sourcery instance.

Root cause

fileAnnotatedContent is instance state populated while processing sourcery:file annotations. In watch mode the same Sourcery instance can run generation more than once, so stale annotated content from a previous pass could be appended again.

Validation

• git diff --check
• xcrun xctest .build/arm64-apple-macosx/debug/SourceryPackageTests.xctest (696 tests, 0 failures)

[krzysztofzablocki]

thanks!