feat: implemented rest of the TCK endpt for FileService.

[Adityarya11]

Description:
Implemented the remainig FileService TCK endpoint.

• AppendFile
• GetFileInfo
• GetFileContents

Related issue(s):

Fixes #1245 #1244 #1243

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

Hey @Adityarya11 👋 thanks for the PR!
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✅ Issue Link -- Linked to #1245 (assigned to you).

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[rwalworth]

Thanks for the contribution @Adityarya11! Great to see the remaining FileService TCK endpoints coming together - the overall structure follows the existing patterns well. I left a few comments below. There are two spots where non-optional fields are used with .has_value() / .value() which would cause compilation errors, plus a handful of copy-paste artifacts to clean up. Once those are addressed we should be good to go!

[rwalworth]

LGTM - nice work addressing all the feedback from the last review, @Adityarya11! The compilation fixes look good, the cleanup came through, and the overall structure follows the existing patterns well.

There are a couple of tiny leftover doc comment nits. Feel free to address them here if you'd like, otherwise I can create follow-up good first issues for them - either way works! I will merge once the workflows pass.