fix(spiffeid): tighten path validation and segment construction

[maxlambrecht]

What

• Require SpiffeId.validatePath() inputs to start with /
• Validate path segments independently when building SPIFFE IDs from segments
• Add tests for accepted non-DNS-shaped trust domains

Why

• Align path validation with segment-based SPIFFE ID construction
• Document and protect trust-domain edge cases allowed by the current SPIFFE spec

How tested

• Updated unit tests in SpiffeIdTest and TrustDomainTest
• Added coverage for path validation, segment-based ID creation, canonical casing, and accepted non-DNS-shaped trust domains

[Copilot]

Pull request overview

This PR tightens SPIFFE ID path validation to align SpiffeId.validatePath() with segment-based SPIFFE ID construction, and expands tests to document trust-domain edge cases allowed by the SPIFFE spec.

Changes:

• Require SpiffeId.validatePath() inputs to start with / and validate segments more explicitly.
• Validate segments independently in SpiffeId.fromSegments(...) (instead of validating as full paths).
• Add/adjust unit tests for canonical casing and non-DNS-shaped trust domains (underscore, IPv4, and other edge shapes).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
java-spiffe-core/src/main/java/io/spiffe/spiffeid/SpiffeId.java	Updates path validation and switches segment construction to validate each segment directly.
java-spiffe-core/src/test/java/io/spiffe/spiffeid/SpiffeIdTest.java	Adds coverage for validatePath leading-slash rules, canonical casing, equality semantics, and non-DNS trust domains.
java-spiffe-core/src/test/java/io/spiffe/spiffeid/TrustDomainTest.java	Adds coverage for non-DNS-shaped trust domains, underscore/IPv4 acceptance, and IPv6 rejection.
java-spiffe-core/src/test/java/io/spiffe/svid/jwtsvid/JwtSvidParseInsecureTest.java	Updates SPIFFE ID construction to use segments ("host") instead of a path string ("/host").

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