[Tests]: add test coverage for generate_mnemonic_from_coin_flips and get_partial_final_word

[S1DDHEY]

Description

Problem or Issue being addressed

generate_mnemonic_from_coin_flips() and get_partial_final_word() in helpers/mnemonic_generation.py had zero test coverage despite being user-facing seed generation paths.

Solution

Added 7 new tests to test_mnemonic_generation.py covering:

1. generate_mnemonic_from_coin_flips():

   • 128 flips - valid 12-word mnemonic
   • 256 flips - valid 24-word mnemonic
   • 4 additional known vectors (all-zeros, all-ones, alternating for both lengths)
   • Random flips always produce valid mnemonics of the correct length
   • Different inputs produce different mnemonics

2. get_partial_final_word():

   • boundary values tested for 7-flip inputs for 12-word mnemonic
   • same for 3-flip inputs for 24-word mnemonic

Additional Information

All tests were cross-verified using emulator with the Tools --> Calc 12th/24th word --> Coin flip entropy flow.
Part of Summer of Bitcoin

Screenshots

N/A, no UI changes.

---

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

---

Checklist

I ran pytest locally

• All tests passed before submitting the PR
• I couldn't run the tests
• N/A

---

I included screenshots of any new or modified screens

Should be part of the PR description above.

• Yes
• No
• N/A

---

I added or updated tests

Any new or altered functionality should be covered in a unit test. Any new or updated sequences require FlowTests.

• Yes
• No, I’m a fool
• N/A

---

I tested this PR hands-on on the following platform(s):

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Emulator

---

I have reviewed these notes:

• Keep your changes limited in scope.
• If you uncover other issues or improvements along the way, ideally submit those as a separate PR.
• The more complicated the PR, the harder it is to review, test, and merge.
• We appreciate your efforts, but we're a small team of volunteers so PR review can be a very slow process.
• Please only "@" mention a contributor if their input is truly needed to enable further progress.

• I understand

---

Thank you! Please join our Devs' Telegram group to get more involved.

[aphrodoe]

All tests verified correct. I independently recomputed expected mnemonics and partial words against embit.
ACK as it is tests only, no production code changes.

[newtonick]

Does this PR increase code coverage (https://github.com/SeedSigner/seedsigner/tree/dev/tests#generate-coverage-manually) ? If so, can the diff of percentage increase or line number increase be posted?

[S1DDHEY]

Yes, this PR increases code coverage for mnemonic_generation.py.

Before

After

This was already addressed earlier, the latest commit adds a bit more edge cases