fix(NATSRS-009-2): CU-86akbhh82 Public Auth struct and constructor lack doc comments

[flamingo]

Closes findings from rule NATSRS-009-2 — Public Auth struct and constructor lack doc comments.

Draft — this is a starting point, not a finished change. The fix required judgment, so read it before trusting it.

#	Fix confidence	Finding	Location
1	🟢 90 high	Public Auth struct and constructor lack doc comments	async-nats/src/auth.rs:3

What changed — and what was deliberately left — is explained per finding as inline review comments on the lines each finding touched.

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Run: https://product-hub.flamingo.so/admin/code-review
Run id: 4fa2224e-8528-443d-a51a-823c78308977

Merging this PR is recorded as acceptance of the rule that produced it;
closing it unmerged is recorded as rejection. Both feed rule health, so
closing a wrong suggestion is useful rather than merely tidy.

ClickUp task: CU-86akbhh82 NATS async-nats and CI review findings (9 PRs)

[flamingo]

🦩 What this fix changed, finding by finding

1 finding(s) fixed in this draft — 1 explained inline on the diff.

[flamingo]

🦩 🟠 Public Auth struct and constructor lack doc comments

Added /// doc comments to the Auth struct, its public fields (jwt, nkey, signature, username, password, token), and Auth::new() in async-nats/src/auth.rs. The private signature_callback field was left undocumented as it is not pub and thus outside the scope of NATSRS-009-2.

🤖 Prompt for AI agents

In async-nats/src/auth.rs around line 3, review and complete this code-review fix: Public Auth struct and constructor lack doc comments.
What the draft fix changed: Added `///` doc comments to the `Auth` struct, its public fields (`jwt`, `nkey`, `signature`, `username`, `password`, `token`), and `Auth::new()` in `async-nats/src/auth.rs`. The private `signature_callback` field was left undocumented as it is not `pub` and thus outside the scope of NATSRS-009-2.
Verify the change is correct and complete; do not refactor unrelated code.

fix confidence: 🟢 90 high — react 👍/👎 to teach the reviewer