Fix Kali bootstrap handling for adb and managed PATH

[Codeblin]

Summary

Fix Kali/Linux bootstrap behavior for managed adb, Frida PATH checks, platform-tools downloads, and GitHub API rate-limit guidance.

This PR makes dexmachina fix --bootstrap behave correctly when tools are installed into DexMachina’s managed workspace instead of the global shell PATH.

Testing

• python -m pytest
• dexmachina --help
• dexmachina status --offline

Additional validation:

• py -m pytest -> 135 passed
• Focused tests for direct installs, device helpers, and doctor checks passed

Device or Emulator Coverage

If this affects device flows, note what you tested:

• Setup: Kali/Linux-style managed install path behavior reproduced from user output
• Rooted: Not required for this change
• Command: dexmachina fix --bootstrap
• Result: Managed adb is now resolved from .dexmachina/tools/adb/bin/adb; Frida checks use DexMachina’s managed environment; fix re-check reloads config after active Frida venv changes

Supply Chain Impact

• No new downloads or network calls
• New/changed downloads are documented in THREAT_MODEL.md
• Checksums/signatures were added when upstream provides them

Notes:

• Platform-tools download validation still fails closed on short/incomplete downloads.
• Downloads that are larger than Content-Length are now accepted to handle decoded transfer-size behavior from Google platform-tools responses.
• GitHub API rate-limit failures now tell users to set GITHUB_TOKEN.