feat(transport)!: replace the legacy transport API

[giortzisg]

Description

Replace the legacy transport path with envelope-based delivery, supporting built-in and custom transports consistently.

Issues

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Reminders

• Add GH Issue ID & Linear ID
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

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Changelog Entry

• Replace the event-based transports with envelope-based ones. Custom transports need to implement the new Transport interface. See the transport section of the migration guide for details.
  • Custom transports need to implement SendEnvelope(ctx, envelope) in place of SendEvent(event), and FlushWithContext(ctx) in place of Flush(timeout). The envelope types live in the new public protocol package.
  • The transport Timeout now applies to every request, including when a custom HTTPClient is set.
  • ClientOptions.DisableTelemetryBuffer is removed. The telemetry buffer is always used.
  • Transport BufferSize now counts envelopes instead of events, and defaults to 1000 instead of 30.

// Before
func (t *customTransport) SendEvent(event *sentry.Event) {}
func (t *customTransport) Flush(timeout time.Duration) bool { return true }

// After
func (t *customTransport) SendEnvelope(ctx context.Context, envelope *protocol.Envelope) error { return nil }
func (t *customTransport) FlushWithContext(ctx context.Context) bool { return true }

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