Add SPIFFE interoperability, security, and end-to-end coverage

[jhrozek]

Description

Verify both authentication methods against real SPIRE infrastructure and
ToolHive's deployed configuration, and document the threat model and
operational failure modes.

Unit tests with hand-rolled certificates and self-signed JWTs will not catch
the failures that matter here. The interesting bugs live in the gap between what
SPIRE actually issues and what the validator assumes — SAN shapes, key usage,
bundle rotation timing, audience handling — so the positive paths must run
against credentials SPIRE minted.

Scope

• Add table-driven parser, validator, policy and client-authentication tests.
• Add real TLS and JWT-SVID integration tests.
• Add an end-to-end SPIRE deployment and test matrix.
• Document the threat model, supported configuration, rotation behaviour and
  operational failure modes.

Acceptance criteria

Positive paths, against real SPIRE-issued credentials:

• X.509-SVID and JWT-SVID authentication both succeed.
• Both methods produce equivalent grants and Cedar authorization decisions for
  the same SPIFFE ID.

Negative and boundary coverage:

• Malformed and additional URI SANs
• Invalid X.509 chains and wrong key usage
• Wrong trust domains
• Missing or mismatched client IDs
• Expired JWT assertions
• Incorrect or multiple JWT audiences
• Wrong JWT keys, key IDs and algorithms
• OIDC token / JWT-SVID type confusion
• Bundle rotation, key removal, and cross-trust-domain mix-up
• Exact and wildcard client-association boundaries
• All four client shapes: public, secret-confidential, X.509-SVID, JWT-SVID
• client_credentials and RFC 8693 token exchange
• Restart and persisted-association behaviour

Discovery:

• Metadata verified for X.509-only, JWT-only and combined configurations.
• Public-only deployments' metadata is unchanged.

Documentation:

• Threat model covers CA or signing-key compromise, Bundle Endpoint compromise,
  assertion replay, TLS termination, and Workload API socket access.
• Production deployment and troubleshooting documentation is complete.

Dependencies

• Sub-issue 6 (SPIRE and operator deployment integration) — the E2E matrix needs
  the deployable path

Related

• Part of the SPIFFE client-authentication epic