Split DaprClientImpl into package-private classes per building block #1875

Describe the feature

sdk/src/main/java/io/dapr/client/DaprClientImpl.java is 2,771 lines. Every building block adds to it, so reviews and rebases keep getting harder.

Proposal: move the method bodies into package-private classes in io.dapr.client, one per building block. Each public method on DaprClientImpl keeps its signature and delegates in one line.

This is not a breaking change:

• DaprClientImpl is public, but every constructor is package-private, so nothing outside io.dapr.client can subclass it.
• All public method signatures stay the same.
• The new classes are package-private, so the public API does not grow.
• DaprClientGrpcTest and DaprPreviewClientGrpcTest test through the public methods and should pass unchanged.

Planned PRs, one per row (line numbers from 00f769f25):

• Shared gRPC plumbing (intercept, createMono, createFlux, stream observers), lines 1759–1833. The other PRs depend on this one. (refactor: move gRPC call plumbing out of DaprClientImpl #1883)
• Conversation (converse, converseAlpha2 and proto mapping), lines 1854–2233 (refactor: move conversation out of DaprClientImpl #1898)
• State (get, bulk, save, delete, transactions, query), lines 305–329, 839–1128, 1295–1377 (refactor: move state management out of DaprClientImpl #1902)
• Pub/sub (publish, bulk publish, subscriptions), lines 383–695 (refactor: move pub/sub out of DaprClientImpl #1905)
• Binary files, lines 2525–2771
• Jobs, lines 1529–1742
• Crypto, lines 2331–2524
• Configuration, secrets, lock and metadata mapping

Constructors, service invocation, bindings and lifecycle stay in DaprClientImpl. The target is under 800 lines.

Release Note

RELEASE NOTE: N/A (internal refactor, no API change)