fix: Make defaultValue optional with a disabled default

[jsonbailey]

Note

Medium Risk
Public SDK API/type changes (optional parameter + new defaulting behavior) could impact consumers’ TypeScript expectations and runtime defaults, though the behavior is straightforward and covered by new tests.

Overview
Updates the Server AI SDK API so defaultValue becomes optional across completionConfig, agentConfig, judgeConfig, createChat, createJudge, and related deprecated/multi-agent helpers; when omitted or null, the SDK now evaluates flags using a shared { enabled: false } disabled fallback.

Adds tests covering the no-default behavior and updates docs/examples to demonstrate omitting defaultValue (or providing one for resiliency) and clarifies the new semantics in README.md and interface/type docs.

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[github-actions]

@launchdarkly/js-sdk-common size report
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Compressed size: 25566 bytes
Compressed size limit: 26000
Uncompressed size: 125383 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 23845 bytes
Compressed size limit: 25000
Uncompressed size: 82750 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 20569 bytes
Compressed size limit: 24000
Uncompressed size: 107197 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 171694 bytes
Compressed size limit: 200000
Uncompressed size: 799867 bytes