Replace MarketDataClient builder with explicit constructors

[MarketDataDev03]

Replace MarketDataClient builder with explicit constructors

Summary

Removes the MarketDataClient.Builder pattern in favour of two public
constructors — a no-arg one for the production happy path (everything resolved
from the §4 cascade) and a 4-arg one for tests and short-lived runtimes that
need explicit control. All fields stay final; the client remains immutable.

Motivation

The previous MarketDataClient.builder()...build() shape carried more
ceremony than the surface warrants: the type has 4 settable knobs, three of
which are routinely left at their default. The team chose to favour a direct
constructor surface — new MarketDataClient() for production and new MarketDataClient(apiKey, baseUrl, apiVersion, validateOnStartup) for explicit
control — over the fluent builder, accepting the trade-off that adding a 5th
knob in the future will require either a new overload or a SemVer bump.

Changes

MarketDataClient (src/main/java/com/marketdata/sdk/MarketDataClient.java)

• Removed the inner Builder class and the static Builder builder() factory.
• Added two public constructors:
  • MarketDataClient() — production constructor. Delegates via this(...) to
    the master constructor with null, null, null, true. All settings come
    from the cascade (env var → .env → built-in default); startup validation
    is enabled.
  • MarketDataClient(@Nullable String apiKey, @Nullable String baseUrl, @Nullable String apiVersion, boolean validateOnStartup) — master
    constructor. Holds the cascade-resolution and initialisation logic. Each
    nullable parameter still falls back to the cascade for that single value.
• Fields are unchanged in shape and remain final. The client is immutable.
• Class-level Javadoc rewritten to describe both constructors and to call out
  that instances are immutable.

Test suite (src/test/java/com/marketdata/sdk/MarketDataClientTest.java)

• Migrated all 5 existing call sites from .builder()...build() to the
  matching constructor invocation.
• Added a new test, noArgConstructorAppliesProductionDefaults, that
  exercises new MarketDataClient() directly and asserts the contract:
  • isValidateOnStartup() is true (env-independent — encoded in the no-arg
    constructor).
  • getUserAgent() starts with marketdata-sdk-java/.
  • getBaseUrl() and getApiVersion() fall back to the documented defaults
    when no MARKETDATA_* env override is set (gated on System.getenv like
    the existing demoModeWhenNoTokenAvailable test).
• The demoModeWhenNoTokenAvailable test now uses new MarketDataClient()
  (its assertion was already valid; the call site just dropped the
  three-null clutter).

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