feat(a2agrpc): add GRPCConnectionPool for gRPC client connection reuse (fixes #109)

[kuangmi-bit]

Summary

Adds a GRPCConnectionPool interface and a default in-memory implementation that enables gRPC connection reuse across client creations. Fixes #109.

Problem

WithGRPCTransport calls grpc.NewClient(url, opts...) on every client creation. gRPC ClientConn is designed to be long-lived — it handles name resolution, TCP reconnection with backoff, and TLS handshakes. Creating one per call is wasteful, especially for integration patterns like ADK RemoteAgent where the constructor opens a connection on every invocation.

Solution

Adds two new exports to a2agrpc/v1:

1. GRPCConnectionPool interface

type GRPCConnectionPool interface {
    Acquire(ctx context.Context, url string, opts ...grpc.DialOption) (*grpc.ClientConn, error)
    Release(*grpc.ClientConn) error
}

2. DefaultGRPCConnectionPool

An in-memory pool keyed by URL with configurable idle TTL. Zero/negative TTL disables eviction.

pool := NewDefaultGRPCConnectionPool(5 * time.Minute)

3. WithPooledGRPCTransport

A FactoryOption that acquires from the pool and releases on Destroy():

pool := a2agrpc.NewDefaultGRPCConnectionPool(5 * time.Minute)
defer pool.Close()

client := a2aclient.NewFromCard(ctx, card,
    a2agrpc.WithPooledGRPCTransport(pool, grpc.WithTransportCredentials(...)),
)

Design decisions

• Pool is explicit (WithPooledGRPCTransport) not implicit — users opt in
• Accepts opts at Acquire time for consistency with WithGRPCTransport
• TTL eviction runs on Acquire to avoid background goroutines
• Release does NOT close the connection — the pool owns the lifecycle
• 6 tests: acquire, reuse, multi-URL, TTL eviction, zero-TTL, release

[gemini-code-assist]

Caution

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[kuangmi-bit]

@nahapetyan-serob — gentle ping on this one too. CI is all green (Go + ITK + PR title).

This adds GRPCConnectionPool for gRPC client connection reuse (fixes #109). Each new connection currently pays the full TLS handshake cost — the pool amortizes this across calls. 34 tests, zero regressions.

Would appreciate a review when you have time.