feat: map the remaining peer connection configuration fields

[devopvoid]

RTCConfiguration gains the settings WebRTC has and the Java API didn't, plus a fix for how createPeerConnection() reports a rejected configuration.

API

Group	New fields
Candidate gathering	iceCandidatePoolSize, tcpCandidatePolicy, candidateNetworkPolicy, continualGatheringPolicy, disableIpv6OnWifi, maxIpv6Networks, networkPreference, vpnPreference, surfaceIceCandidatesOnIceTransportTypeChanged
ICE checks	iceConnectionReceivingTimeout, iceBackupCandidatePairPingInterval, iceCheckIntervalStrongConnectivity, iceCheckIntervalWeakConnectivity, iceCheckMinInterval, iceUnwritableTimeout, iceUnwritableMinChecks, iceInactiveTimeout, stunCandidateKeepaliveInterval, stableWritableConnectionPingInterval, prioritizeMostLikelyIceCandidatePairs, enableIceRenomination
TURN	presumeWritableWhenFullyRelayed, turnPortPrunePolicy, turnLoggingId
Media	enableDscp, enableCpuAdaptation, suspendBelowMinBitrate, screencastMinBitrate
Security	cryptoOptions: the new RTCCryptoOptions for SRTP cipher suites and cryptex (RFC 9335)
Signaling	offerExtmapAllowMixed, enableImplicitRollback, alwaysNegotiateDataChannels

New enums: RTCTcpCandidatePolicy, RTCCandidateNetworkPolicy, RTCContinualGatheringPolicy, RTCPortPrunePolicy, RTCVpnPreference, RTCAdapterType, RTCCryptexPolicy.

Design

• Unset means WebRTC's default. The new fields use nullable types. Several WebRTC defaults are true (DSCP, CPU adaptation, extmap-allow-mixed), and a plain Java boolean would have silently turned them off.
• getConfiguration() returns the values in effect, defaults included.
• RTCAdapterType is mapped explicitly, because the native adapter types are bit flags, which the by-ordinal enum conversion can't map.
• Left out:
  • requireFrameEncryption: it needs a native frame encryptor, which the library doesn't expose (its encryption uses encoded transforms), so it would stop all sending.
  • Experimental or internal fields: SCTP SNAP, pacer burst, usage-pattern reporting.
• Docs checked against WebRTC's comments. For example, networkPreference wins over pair priority and network cost rather than only breaking ties, and screencastMinBitrate pads screen share video up to that bitrate.

Fix (separate commit)

• Garbled error: when WebRTC rejected a configuration, createPeerConnection() threw with a garbled error type. ToString(RTCErrorType) returns a string_view, which was passed to a %s vararg as it was (undefined behavior). It's now passed as a C string.
• Leaked observer: the native observer created for the connection leaked in that case, since no peer connection took it. It's now deleted.

Testing

RTCConfigurationFieldsTests (5 tests):

• Native defaults: a default configuration reads back WebRTC's defaults, including the true ones.
• Round trip: every new field survives creation and getConfiguration().
• setConfiguration(): changes to modifiable fields are applied.
• Rejected configuration: the error message names a readable error type.
• Behavior: with tcpCandidatePolicy = DISABLED, gathering yields no TCP candidates.

mvn -pl webrtc test: 189 tests pass. -Pjni-check: 189 tests pass, with no FATAL ERROR in native method.

Docs

New guide: docs/guide/networking/peer-connection-config.md (Peer Connection Configuration), linked from the sidebar and the guide index.