Add configurable min/max TLS version with hybrid per-transport default

[DanPeterson]

Summary

SPP 9.0 enables TLS 1.3. Go's crypto/tls client cannot answer a TLS 1.3 post-handshake CertificateRequest (verified in the stdlib: handlePostHandshakeMessage only handles newSessionTicketMsgTLS13/keyUpdateMsg; a certificateRequestMsgTLS13 errors with unexpected_message, and Go never advertises post_handshake_auth). Because SPP certificate login and A2A rely on SPP's post-handshake certificate request, they break against a TLS 1.3-only appliance on the Standard binding. Password / PKCE / browser / device-code / token flows are unaffected.

This PR makes the TLS version window configurable and picks a safe hybrid default so cert-auth keeps working out of the box on 9.0.

What changed

New options (tls.go):

• WithMinTLSVersion(v uint16) / WithMaxTLSVersion(v uint16) — a uint16 passthrough to crypto/tls version constants. No custom enum, so a future TLS 1.4 works with zero SDK change. Sub-1.2 is allowed as an explicit escape hatch (guarded #nosec G402).

Hybrid per-transport default (transport.go, tls.go):

• TLS 1.2 floor everywhere.
• serverTrust transport (password/token/PKCE) stays open to 1.3.
• clientCert transport (certificate login, A2A) is capped at 1.2 when no explicit version option is set — so post-handshake cert-auth "just works" on the 9.0 Standard binding.
• Setting either option lifts the cap on both transports, enabling TLS 1.3 cert-auth against the appliance Cert SNI hostname (where the cert is requested in the initial handshake). No new code path needed — the existing clientCert transport already presents the cert up front.

Docs & sample:

• .agents/skills/api-patterns, architecture, a2a-workflow skills + AGENTS.md document the version window, hybrid default, the Cert SNI route, and the 60094 Authorization is denied symptom.
• samples/certificate gains -min-tls / -max-tls flags.

Tests

tls_test.go: TestBuildTLSConfigVersionWindow, TestClientCertMaxTLSResolver, TestClientCertTransportCapsMaxTLS (default caps clientCert to 1.2 while serverTrust stays open; explicit options lift the cap; max < min is rejected). go build, go vet, gofmt -l, golangci-lint run, and go test ./... are all green.

Live validation against a 9.0 appliance is a maintainer step (CI has no appliance): confirm default cert-auth works at effective 1.2 on the Standard binding, WithMaxTLSVersion(tls.VersionTLS13) + Cert SNI hostname enables 1.3 cert-auth, and WithMinTLSVersion(tls.VersionTLS13) fails closed on Standard-binding cert flows.

Relation to SafeguardJava #190

Same root limitation (JSSE also can't do TLS 1.3 client PHA). Differences: Java defaults to TLS 1.2-only for all traffic via a process-wide static enum; this PR keeps 1.3 for non-cert flows via a per-client, per-transport default and a future-proof uint16 window, matching the PySafeguard min/max approach.