test: add comprehensive unit tests for discovery-tags.ts

[DevAnuragT]

Description

This PR introduces comprehensive unit tests for src/transport/discovery-tags.ts.

By establishing 100% test coverage now, this effectively locks down the current capability parsing behavior (used for CEP-15/17/19 discovery) and provides a safe foundation before extending the module to parse the upcoming CEP-15 D/I (defines/implements) schema tags.

Changes Made

• Test Suite Added: Created src/transport/discovery-tags.test.ts co-located with the source file.
• 100% Coverage: Verified 100% line and function coverage across all methods (hasSingleTag, hasEventTag, getDiscoveryTags, parseDiscoveredPeerCapabilities, learnPeerCapabilities, mergeDiscoveryTags).
• Edge Cases Handled: Added defensive checks for undefined inputs, empty arrays, missing tags, and unknown tags.
• Immutability Verified: Added strict assertions to verify array cloning behavior and prevent unintended reference mutations.
• Style Alignment: Followed the established conventions from announcement-manager.test.ts using native bun:test and existing project constants.