Implements SEP-2549 "TTL for List Results", which lets servers attach
optional caching freshness hints to the five cacheable result types:
tools/list, prompts/list, resources/list, resources/templates/list, and
resources/read.

Protocol changes:
- Add ICacheableResult with TimeToLive (serialized as integer-millisecond
  ttlMs) and CacheScope (serialized as cacheScope).
- Add the CacheScope enum (public, private) with lowercase wire values.
- Implement the interface on the five cacheable result types.
- Register CacheScope for source-generated serialization.

Both fields are optional and omitted when unset, so the change is fully
backward compatible and requires no capability negotiation. The SDK
propagates the values without consuming them.

Robustness and security:
- ttlMs deserialization clamps out-of-range, fractional, and overflowing
  values (including positive and negative infinity) to TimeSpan.MinValue
  or MaxValue instead of throwing, so a malformed or hostile hint cannot
  break reading of the enclosing result. The shared
  TimeSpanMillisecondsConverter uses the non-throwing TryGetDouble and
  clamps by token sign, giving identical behavior on .NET and on .NET
  Framework (whose number parser reports failure on overflow rather than
  returning infinity).
- cacheScope deserialization tolerates unknown or future values by mapping
  them to null (treated as the public default) instead of failing the whole
  result, and matches the known values case-insensitively so a mis-cased
  "private" is honored rather than silently downgraded to public.

Tests:
- Serialization, round-trip, omission, and clamping edge cases for ttlMs.
- Unknown, partial, and case-insensitive cacheScope handling.
- Per-page independence of caching hints for pagination.
- End-to-end propagation of hints from server to client.
- Regression coverage for the shared converter used by McpTask ttl and
  pollInterval.
- Caching conformance scenario wiring, gated to the conformance build that
  provides it.

Verified across net8.0, net9.0, net10.0, and net472, and under Native AOT
publish with no trimming or AOT warnings.

- CacheScopeConverter.Read now consumes non-string tokens with reader.Skip()
  before returning null. Previously an object or array value for cacheScope
  left the reader mispositioned and threw "read too much or not enough",
  breaking deserialization of the whole result. Added object and array cases
  to the tolerant-deserialization test.
- GetInstalledConformanceVersion no longer calls EnsureNpmDependenciesInstalled.
  The version check backs Theory skip gates and must be side-effect-free; it
  now returns null when the conformance package is absent. The actual scenario
  run path still restores npm dependencies via ConformanceTestStartInfo.

Results that carry caching hints (tools/list, prompts/list, resources/list,
resources/templates/list, resources/read) now always emit ttlMs and cacheScope
on the wire. When a handler leaves them unset, the server fills in conservative
defaults (ttlMs: 0, cacheScope: private) so the required fields are present
while preserving today's 'don't cache' behavior. Handler- or filter-supplied
values are left untouched. The properties remain nullable so the client can
still represent their absence from older or non-conformant servers.

The auto-paginating ListToolsAsync/ListPromptsAsync/ListResourcesAsync/
ListResourceTemplatesAsync overloads aggregate all pages into a single list
and do not surface the per-result ttlMs/cacheScope hints. Add a remarks note on
each pointing callers at the raw single-page overload that returns the result
type carrying those hints.

…Converter

The upstream merge brought in CP0001 baseline suppressions for
TimeSpanMillisecondsConverter (the Tasks rework had deleted the type). This
branch keeps the type for the SEP-2549 cacheable results, so those suppressions
are now unnecessary and fail package validation (make pack) in Release. Drop the
four stale entries so ApiCompat passes.