fix(protoapp): propagate binding errors to plugins via lastError API

[pabloinigoblasco]

Summary

When ensureParserBinding() fails, plugins need to know why it failed to show meaningful error messages. The SDK provides the lastError() API for this purpose, but the protoapp implementation always returned nullptr.

Before this fix

Plugins received empty error strings:

No channels could be bound to parsers:
  - /camera/image (encoding: ros1):

Users couldn't distinguish between:

• "Parser not installed" (they can install it)
• "Schema corrupted" (the file has a problem)
• "Internal parser error" (a bug)

After this fix

Plugins show actionable messages:

No channels could be bound to parsers:
  - /camera/image (encoding: ros1): no parser found for encoding 'ros1'
  - /bad/data (encoding: cdr): failed to parse broken_msgs/Garbage: Missing ROSType

Technical details

The SDK uses a C ABI for plugin ↔ host communication. When ensure_parser_binding() returns false, the SDK wrapper calls get_last_error() to populate the Expected<Handle> error message. This contract was already designed correctly — the bug was that rhGetLastError() always returned nullptr.

Changes

File	Change
data_source_session.hpp	Add std::string last_error to RuntimeHostState
data_source_session.cpp	Store error message at each of 5 failure points
data_source_session.cpp	Return state->last_error from rhGetLastError()

Failure points now covered

1. No parser found for encoding
2. Failed to create parser instance
3. Failed to create topic in datastore
4. Failed to bind write host to parser
5. Failed to parse schema (includes parser's lastError)

Impact

Enables MCAP, Foxglove Bridge, and PJ Bridge plugins to report binding failures with context. No SDK changes required.

Test plan

• Load MCAP with unsupported encoding → verify error message shows encoding name
• Load MCAP with malformed schema → verify error message shows parse failure details