Port upstream 0.64.1: Neuralwatt retry, validation and endpoint parity

[Finesssee]

Summary

Brings the native Neuralwatt provider (rust/src/providers/neuralwatt/) in line with the behavior upstream keeps after moving Neuralwatt to the bundled neuralwatt.js plugin in v0.64.1:

• One retry of a transient quota failure: HTTP 408/429/500/502/503/504, and timeout, connection lost/reset, connection refused and DNS errors. The delay is Retry-After (non-negative number, capped at 10 s) or 1 s. There is no second retry; the final failure reads Neuralwatt API error: HTTP <status> (previously HTTP 503 Service Unavailable). 401/403 stay AuthRequired and are never retried, TLS handshake failures are never retried, and cancelling the fetch (refresh timeout) drops the retry sleep with the future.
• SubscriptionMetadata.renews_at is set from subscription.current_period_end unless subscription.auto_renew == false.
• Parse failure when balance is missing, or when none of credits_remaining_usd >= 0, credits_used_usd >= 0, total_credits_usd > 0 is usable. Present-but-invalid timestamps and wrong-typed fields (including usage.*.requests/tokens non-integers) are parse failures, as in upstream's schema checks.
• The login-method fallback title-cases accounting_method (energy becomes Energy) whenever there is no plan, including when subscription is null (previously it only applied inside a non-null subscription and was not capitalized). Plan and key-allowance labels use the same upstream title casing.
• NEURALWATT_API_URL must be HTTPS or a bare host (promoted to HTTPS), otherwise the fetch fails with Neuralwatt endpoint override NEURALWATT_API_URL must use HTTPS or a bare host. A configured ?query is kept after /v1/quota. Validation reuses the shared providers::validated_https_url.
• The subscription reset date is now kept when current_period_start is absent (upstream sets resetsAt from the end alone); the window length is still only set when both ends are present.

Retry logic is provider-local (no shared retry layer). The retry delay is an argument of the private fetch_quota, so tests run without waiting.

Upstream reference

• v0.64.1 release: "Neuralwatt: use the bundled quota plugin while preserving subscription usage, prepaid balances, key allowances, selective retries and cancellation" (refactor(providers): move configured proxy usage into plugins steipete/CodexBar#3811, 18ee35247).
• Tag-pinned sources (v0.64.1): Sources/CodexBarCore/Resources/Plugins/neuralwatt.js, Sources/CodexBarCore/ProviderHTTPClient.swift (ProviderHTTPRetryPolicy.transientIdempotent), Sources/CodexBarCore/Providers/NeuralWatt/NeuralWattSettingsReader.swift (validateEndpointOverrides), NeuralWattProviderDescriptor.swift, TestsPlugin/NeuralWattPluginTests.swift (fixtures and malformed-body list), docs/neuralwatt.md.

Ported / Deferred

Ported: everything listed in Summary.

Not ported (no Windows counterpart or already matching): the plugin-engine migration itself (Swift/QuickJS), the cost shape (upstream reports the balance as used/limit: 0; Windows keeps CostSnapshot::with_balance with used/limit), and percent-decoding of the base path before the /v1 check. Connection-closed detection uses the hyper message connection closed before message completed because hyper is not a direct dependency. No deferrals of wire shapes.

Validation

Run in this worktree with a process-local CARGO_TARGET_DIR outside every worktree (equivalent to scripts/worktree-env.ps1; the script cannot run under this agent's worktree guard because it calls git):

• cargo fmt --all: clean.
• cargo test -p codexbar neuralwatt: 18 passed, 0 failed (upstream quota fixtures 0-9, malformed bodies, auto_renew true/false/absent, endpoint override, retry exactly 2 requests for 503 and 429-then-200, no retry for 400/401/403/404, timeout and lost-connection retry on a raw-socket stub, TLS handshake not retried, retry sleep dropped when the fetch future is cancelled).
• cargo clippy -p codexbar --all-targets -- -D warnings: no findings in neuralwatt. It reports 3 pre-existing errors in files this PR does not touch (alibabatokenplan/cli.rs:163, kiro/usage_limits.rs:315, openai/subscription.rs:206; new lints in the installed rustc 1.96). They are also present on the base branch.
• Not run: full cargo test -p codexbar (no shared code touched), Tauri crate tests (not touched).

Affected areas

• Rust backend / provider (Neuralwatt only)
• Tauri shell / frontend / tray / settings / float bar
• Docs

UI proof

Not applicable. No UI surface changed; the renewal date renders through the existing subscription-metadata path.

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[Finesssee]

Validation on the CI-pinned toolchain (Rust 1.98.0, scripts/circleci-pinned-rust.txt), run at head c24373b0:

• cargo +1.98.0 fmt --all --check: clean
• cargo +1.98.0 clippy --workspace --all-targets -- -D warnings: clean, including the Tauri crate
• cargo +1.98.0 test -p codexbar neuralwatt: 18 passed, 0 failed

The three clippy errors mentioned in the PR body are in files this PR doesn't touch. They only appear on the machine's older default toolchain (1.96.1); main and this branch are both clean on 1.98.0.