Add first-class extended agent card discovery API

[MisterVVP]

Motivation

• Provide explicit support for fetching the extended Agent Card view so the client can request the richer discovery payload used by other SDKs and parity tasks.
• Keep discovery behavior consistent with the existing Fetch flow (validation, error mapping, parsing) while offering a dedicated API for the extended endpoint.

Description

• Added a public API DiscoveryClient::FetchExtendedAgentCard(std::string_view) in include/a2a/client/discovery.h to expose the extended discovery flow.
• Implemented DiscoveryClient::FetchExtendedAgentCard and BuildExtendedDiscoveryUrl in src/client/discovery.cpp that construct the extended discovery endpoint as /.well-known/agent-card.json?view=extended, fetch, parse, and validate the returned AgentCard.
• Added a unit test FetchExtendedAgentCardUsesExtendedQueryPath in tests/unit/discovery_test.cpp that verifies the extended URL is used and that a valid response is parsed and validated.

Testing

• Ran formatting checks via clang-format --dry-run --Werror and normalized code with ./scripts/run_clang_format.sh (passed).
• Ran the full repository verification flow ./scripts/verify_changes.sh which performed CMake configure/build, ctest, and clang-tidy; the workflow completed and returned success.
• Executed the test suite via ctest during verification and observed all tests passed (128/128 tests).
• Ran ./scripts/run_clang_tidy.sh build as part of verification and observed it completed successfully.

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Codex Task