Add Media3 session foundation

[sztomek]

Description

This is the first step of the media3 session migration.
My plan is to raise incremental PRs, each building on the previous one.
Merge order should be respected: this will be merged last once every change is here and tested.

Testing Instructions

Just review the code.

Checklist

• If this is a user-facing change, I have added an entry in CHANGELOG.md
• Ensure the linter passes (./gradlew spotlessApply to automatically apply formatting/linting)
• I have considered whether it makes sense to add tests for my changes
• All strings that need to be localized are in modules/services/localization/src/main/res/values/strings.xml
• Any jetpack compose components I added or changed are covered by compose previews
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[Copilot]

Pull request overview

Lays groundwork for an incremental migration to Media3 MediaSession by introducing a feature flag, starting to decouple cast playback state from the legacy compat constants, and adding the Media3 session dependency.

Changes:

• Added Feature.MEDIA3_SESSION remote-config/dev-toggle feature flag.
• Replaced PlaybackStateCompat integer state usage in CastPlayer with an internal enum.
• Added the androidx.media3:media3-session dependency to the repositories module (and version catalog).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
modules/services/utils/src/main/java/au/com/shiftyjelly/pocketcasts/utils/featureflag/Feature.kt	Adds a feature flag entry for gating the Media3 session migration.
modules/services/repositories/src/main/java/au/com/shiftyjelly/pocketcasts/repositories/playback/CastPlayer.kt	Refactors cast playback state tracking away from PlaybackStateCompat constants.
modules/services/repositories/build.gradle.kts	Adds Media3 session dependency to repositories’ exported deps.
gradle/libs.versions.toml	Defines media3-session in the version catalog.

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[wpmobilebot]

Project dependencies changes

list

+ New Dependencies
androidx.media3:media3-session:1.9.3

! Upgraded Dependencies
androidx.media:media:1.7.0, (changed from 1.6.0)

tree

 +--- androidx.mediarouter:mediarouter:1.8.1
-|    \--- androidx.media:media:1.4.1 -> 1.6.0
-|         +--- androidx.annotation:annotation:1.2.0 -> 1.9.1 (*)
-|         +--- androidx.collection:collection:1.1.0 -> 1.5.0 (*)
-|         \--- androidx.core:core:1.6.0 -> 1.18.0 (*)
+|    \--- androidx.media:media:1.4.1 -> 1.7.0
+|         +--- androidx.annotation:annotation:1.2.0 -> 1.9.1 (*)
+|         +--- androidx.collection:collection:1.1.0 -> 1.5.0 (*)
+|         +--- androidx.core:core:1.6.0 -> 1.18.0 (*)
+|         \--- androidx.core:core:1.9.0 -> 1.18.0 (*)
++--- androidx.media3:media3-extractor:1.9.3
+|    \--- androidx.media3:media3-decoder:1.9.3
+|         \--- androidx.media3:media3-common:1.9.3
+|              \--- androidx.media3:media3-session:1.9.3 (c)
 +--- com.google.android.gms:play-services-cast-framework:22.3.0
-|    \--- androidx.media:media:1.6.0 (*)
+|    \--- androidx.media:media:1.6.0 -> 1.7.0 (*)
 +--- project :modules:features:account
 |    \--- project :modules:features:search
 |         \--- project :modules:services:analytics
 |              \--- project :modules:services:model
 |                   \--- project :modules:services:utils
 |                        \--- androidx.car.app:app:1.7.0
-|                             \--- androidx.media:media:1.6.0 (*)
+|                             \--- androidx.media:media:1.6.0 -> 1.7.0 (*)
+|         \--- project :modules:services:repositories
+|              \--- androidx.media3:media3-session:1.9.3
+|                   +--- androidx.media3:media3-datasource:1.9.3 (*)
+|                   +--- androidx.collection:collection:1.2.0 -> 1.5.0 (*)
+|                   +--- androidx.media:media:1.7.0 (*)
+|                   +--- androidx.core:core:1.8.0 -> 1.18.0 (*)
+|                   \--- androidx.media3:media3-common:1.9.3 (*)