feat: add captureException() for error tracking

[hnipps]

Summary

Adds captureException() to the PHP SDK, enabling PHP applications to send exceptions to PostHog's error tracking — matching the existing Python and Node SDK implementations.

• New ExceptionUtils class — converts \Throwable to $exception_list payload with stack frames, source context, and chained exception support
• Client::captureException() — captures exceptions as $exception events with configurable source context lines
• PostHog::captureException() — static facade for the same functionality
• Anonymous capture — when no distinctId is provided, generates a UUID v4 and sets $process_person_profile: false

Key design decisions

• No fingerprint generation — PostHog backend handles grouping (matches Python/Node)
• No autocapture (set_exception_handler) — keeping initial PR scoped to manual capture; autocapture can follow in a separate PR
• No args in frames — PHP trace includes function args but they can contain sensitive data and blow batch size limits
• platform: "php" — matches Python's "python" and Node's "node:javascript" convention
• Source context default 5 lines — matches Python SDK; configurable via exception_source_context_lines option (0 to disable)
• Outermost-first ordering — chained exceptions are listed with outermost first, root cause last

Usage

// With distinct ID
PostHog::captureException($exception, 'user-123');

// Anonymous (generates UUID, disables person processing)
PostHog::captureException($exception);

// With additional properties
PostHog::captureException($exception, 'user-123', [
    'environment' => 'production',
    'release' => 'v1.2.3',
]);

// Unhandled exception
PostHog::captureException($exception, 'user-123', [], false);

// Disable source context
PostHog::init('your-api-key', [
    'exception_source_context_lines' => 0,
]);

Test plan

• 20 new tests covering: basic capture, chained exceptions, anonymous IDs, handled/unhandled, frame ordering, in_app detection, source context on/off, PHP \Error support, static facade, additional properties, 3-deep chains
• All 175 existing tests pass (zero regressions)

Closes PostHog/posthog#31076

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