Add debug-level summary log of back-compatibility changes in http-client-csharp generator

[Copilot]

Back-compatibility adjustments driven by a library's LastContractView (parameter reorders, renamed/preserved params, collection type preservation, constructor promotions, enum reorders, added api-version members, model-factory shims, etc.) were hard to spot in generator run logs. This adds a single human-readable summary listing every such change at the end of generation, emitted through the existing logging pipeline.

Changes

• Extended Emitter (Microsoft.TypeSpec.Generator.EmitterRpc) with categorized, buffered logging:
  • New overloads Info(string, BackCompatibilityChangeCategory), Debug(string, BackCompatibilityChangeCategory), Verbose(string, BackCompatibilityChangeCategory) that buffer messages and dedup them per (level, category).
  • New WriteBufferedMessages() API that emits a single grouped trace per level via the existing JSON-RPC channel.
  • Buffered messages are also auto-flushed on Emitter.Dispose as a safety net.
  • Header wording uses conditional singular/plural based on the count (e.g. 1 message across 1 category vs N messages across M categories).
• New BackCompatibilityChangeCategory enum (Microsoft.TypeSpec.Generator.EmitterRpc): strongly-typed set of categories used by the buffered overloads; display names are mapped centrally inside Emitter.
• CSharpGen.ExecuteAsync: calls Emitter.WriteBufferedMessages() after all ProcessTypeForBackCompatibility runs.
• Instrumented back-compat sites to record their decisions (each call now goes through Emitter.Debug/Info with a BackCompatibilityChangeCategory value):
  • ClientProvider — method parameter reordering
  • RestClientProvider — paging param name preservation (top, maxPageSize casing)
  • ScmModelProvider — AdditionalProperties shape preservation
  • ModelProvider — collection property type preservation; abstract-model ctor modifier promotion
  • ModelFactoryProvider — back-compat method add / replace / skip
  • FixedEnumProvider — enum member reordering
  • ApiVersionEnumProvider — api version members carried over from last contract
• Unit tests added to EmitterTests covering buffering, dedup, null/empty handling, level separation, and reset.

Example output

Summary of grouped 'debug' messages: 4 messages across 3 categories.
  Method Parameter Reordering (2):
    - Reordered parameters of 'FooClient.GetData' to match last contract.
    - Reordered parameters of 'FooClient.UpdateData' to match last contract.
  Parameter Name Preserved (1):
    - Preserved parameter name 'top' on 'FooClient' from last contract (instead of 'maxCount').
  Collection Property Type Preserved (1):
    - Changed property 'Bar.Items' type to 'IReadOnlyList<string>' to match last contract.

Back-compat summaries are emitted at debug level, so they are surfaced only when logLevel: debug (or verbose) is set on the emitter.

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commit: dbd3969