fix: harden inputs and prepare v0.1.1

[airouter-dev]

Summary

• reject invalid UTF-8 and non-printing Unicode model IDs before a chat request
• reject authenticated API keys containing CR or LF before request construction
• add explicit redirect, TLS, no-auth, input-boundary, and CLI version coverage
• test the Go 1.22.0 minimum on Ubuntu and Go 1.26.5 across Ubuntu, macOS, and Windows with pinned Actions, race, formatting, vet, and a pinned govulncheck
• replace provider promotions and uninitialized mirror claims with complete vendor-neutral Chinese and English documentation
• add an accurate threat-model update, security policy notes, and a v0.1.1 changelog

Validation

• GOTOOLCHAIN=go1.22.0 go test -count=1 ./...
• GOTOOLCHAIN=go1.22.0 go vet ./...
• GOTOOLCHAIN=go1.26.5 go test -count=1 ./...
• GOTOOLCHAIN=go1.26.5 go test -race -count=1 ./...
• GOTOOLCHAIN=go1.26.5 go test -shuffle=on -count=20 ./...
• GOTOOLCHAIN=go1.26.5 go vet ./...
• GOTOOLCHAIN=go1.26.5 go run golang.org/x/vuln/cmd/[email protected] ./... (No vulnerabilities found.)
• GOTOOLCHAIN=go1.26.5 go run github.com/rhysd/actionlint/cmd/[email protected]
• git diff --check
• targeted high-confidence secret scan: zero findings

The repository CI and all local tests use only local fixtures; they do not call a real model API. The existing v0.1.0 tag remains unchanged. After merge, this change is intended for v0.1.1.