Investigate source file upload duplication

[szokeasaurusrex]

Description

Adds investigation.md detailing whether sentry-cli can upload source files twice during sourcemap uploads. The investigation concludes that duplication can occur when original .js source files are present in the upload directory alongside sourcemaps that embed their sourcesContent. It also outlines scenarios where this happens and where it doesn't, along with technical details and potential solutions.

Issues

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:no_entry_sign:	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

### Features

- Investigate source file upload duplication ([#2977](https://github.com/getsentry/sentry-cli/pull/2977))

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