chore(vault): make pre-initialization access consistently return NotInitialized

[mac-dubem]

Summary

• Changed get_admin, get_adapter, and require_admin to return ContractError::NotInitialized instead of panicking with .unwrap() when called before initialize()
• Updated set_paused, set_admin, set_adapter, deposit, withdraw, and get_total_assets to propagate the NotInitialized error
• Added 8 new tests covering the uninitialized case for each affected function

This makes error handling consistent — callers and off-chain indexers can match on ContractError::NotInitialized instead of special-casing which functions panic.

Test plan

• cargo clippy --all-targets -- -D warnings passes
• cargo test passes (32 vault tests, all passing)

Closes #375

[vercel]

@mac-dubem is attempting to deploy a commit to the Collins' projects Team on Vercel.

A member of the Team first needs to authorize it.

[collinsezedike]

Verified independently: checked this out, ran cargo clippy --all-targets -- -D warnings (clean) and cargo test -p meridian-vault --lib (32 passed, matches the PR's own claim). The ?-propagation is threaded correctly through every caller, and existing non-try_ test call sites still compile since Soroban's generated client wrapper auto-unwraps Result and panics on Err. Logic is correct.

Two things need fixing before merge.

PR Title check is failing. The title kept the bracketed issue-style format instead of conventional-commit format (type(scope): description). Since this repo squashes on merge, the PR title becomes the final commit message.

This branch predates #397 by about 33 minutes, which gitignored packages/contracts/**/test_snapshots/ and removed all tracked snapshot files. This PR still has 54 tracked snapshot files, including 10 new ones for the tests it adds. Please rebase onto current main and drop all of them from the diff, they're gitignored now and shouldn't be committed.

[collinsezedike]

Hey @mac-dubem, checking in on this one since it's been a few days quiet.

Are you still good to continue?

If you're stuck on anything, let me know here and I can help.

If you're no longer able to finish it, just say so and I'll pass the issue on to someone else.

[collinsezedike]

CI is failing on Commit Messages: the header is 79 characters, over the 72-char limit. Since this repo squash-merges using the PR title, both the commit header and the PR title need to be trimmed to 72 characters or under, e.g. chore(vault): return NotInitialized instead of panicking pre-init.