Preserve unset fields when merging retry options

[Anthounn]

What changed?

• Skip the initial-interval and backoff-coefficient setters when their merged values are unset.
• Add regression coverage for empty and partial policies, override precedence, deferred defaults, and subsequent @MethodRetry merging.
• Add a changelog entry and clarify that merging does not apply defaults.

Why?

RetryOptions.Builder.build() permits partial policies, but RetryOptions.merge(RetryOptions) unconditionally passed their unset values to setters that reject them. For example:

RetryOptions.newBuilder().setMaximumAttempts(3).build()
    .merge(RetryOptions.newBuilder().setDoNotRetry("PermanentFailure").build());

This throws NullPointerException for the unset initial interval. When an initial interval is supplied but neither policy sets a coefficient, merging instead throws IllegalArgumentException for coefficient 0.0.

Preserving the unset values allows callers to compose retry policies before applying defaults or annotation values, without changing precedence for explicitly configured fields.

Validation

• Before the fix: 4 failures and 3 passes in RetryOptionsTest, reproducing both exceptions.
• After the fix: all 8 RetryOptionsTest tests pass, including the 6 new tests.
• ./gradlew --offline :temporal-sdk:spotlessApply and git diff --check pass.
• Broader run: 143 passed, 2 skipped, no failures/errors across 20 suites (io.temporal.common.*, ActivityOptionsTest, and RetryOptionsUtilsTest).
  Two existing ActivityClientCallsInterceptorChainTest tests were skipped.
• Windows, Java 21; full multi-module suite, external Temporal server, and Cloud tests not run.

Breaking changes?

None. No API signatures change; existing explicit-value precedence and null-override behavior are preserved.

Server PR

Not required.

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