[mesh-forwarder] finalize message direct tx on drop or eviction

[abtink]

This commit adds FinalizeMessageDirectTx(), which clears the DirectTx flag on a given message, updates the IPv6 counter, and signals other modules about the transmission status(particularly for MleDiscoverRequest and MleChildIdRequest messages so their internal state can be updated).

This commit ensures FinalizeMessageDirectTx() is called in various scenarios:

• Successful message delivery (all fragments reach destination).
• Any fragment transmission failure (frame tx failure)
• Message drop due to address query failure or malformed message.
• Message drop by queue management.
• Message eviction to prioritize higher-priority messages.

This commit also updates DiscoverScanner to stop an ongoing discover scan if the "MLE Discover Request" message transmission fails (with error other than CSMA error). This is necessary because the DiscoverScanner reuses the same Message instance for tx on different scan channels. If the Message is freed (e.g., evicted), the Message cannot be reused.

Finally, this commit renames RemoveMessage() to EvictMessage() to clarify the purpose and usage of this method.

[size-report]

Size Report of OpenThread

Merging #9682 into main(3754174).

name	branch	text	data	bss	total
ot-cli-ftd	main	463440	760	66244	530444
	#9682	463520	760	66244	530524
	+/-	+80	0	0	+80
ot-ncp-ftd	main	434140	760	61464	496364
	#9682	434220	760	61464	496444
	+/-	+80	0	0	+80
libopenthread-ftd.a	main	233062	0	40198	273260
	#9682	233156	0	40198	273354
	+/-	+94	0	0	+94
libopenthread-cli-ftd.a	main	56204	0	8067	64271
	#9682	56204	0	8067	64271
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	31841	0	5916	37757
	#9682	31841	0	5916	37757
	+/-	0	0	0	0
ot-cli-mtd	main	362648	760	51132	414540
	#9682	362728	760	51132	414620
	+/-	+80	0	0	+80
ot-ncp-mtd	main	345852	760	46376	392988
	#9682	345932	760	46376	393068
	+/-	+80	0	0	+80
libopenthread-mtd.a	main	156442	0	25110	181552
	#9682	156514	0	25110	181624
	+/-	+72	0	0	+72
libopenthread-cli-mtd.a	main	39129	0	8043	47172
	#9682	39129	0	8043	47172
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24721	0	5916	30637
	#9682	24721	0	5916	30637
	+/-	0	0	0	0
ot-cli-ftd-br	main	531040	768	130900	662708
	#9682	531120	768	130900	662788
	+/-	+80	0	0	+80
libopenthread-ftd-br.a	main	295566	5	104830	400401
	#9682	295658	5	104830	400493
	+/-	+92	0	0	+92
libopenthread-cli-ftd-br.a	main	69552	0	8091	77643
	#9682	69552	0	8091	77643
	+/-	0	0	0	0
ot-rcp	main	62168	564	20604	83336
	#9682	62168	564	20604	83336
	+/-	0	0	0	0
libopenthread-rcp.a	main	9522	0	5052	14574
	#9682	9522	0	5052	14574
	+/-	0	0	0	0
libopenthread-radio.a	main	18811	0	214	19025
	#9682	18811	0	214	19025
	+/-	0	0	0	0

[codecov]

Codecov Report

Merging #9682 (9bfdd8b) into main (3754174) will increase coverage by 0.00%.
The diff coverage is 80.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9682   +/-   ##
=======================================
  Coverage   85.69%   85.70%           
=======================================
  Files         555      555           
  Lines       73753    73765   +12     
=======================================
+ Hits        63203    63218   +15     
+ Misses      10550    10547    -3     

Files	Coverage Δ
src/core/thread/discover_scanner.hpp	50.00% <ø> (ø)
src/core/thread/mesh_forwarder.hpp	72.72% <ø> (ø)
src/core/thread/discover_scanner.cpp	81.48% <87.50%> (-0.34%)	⬇️
src/core/thread/mesh_forwarder_ftd.cpp	85.71% <66.66%> (+0.52%)	⬆️
src/core/thread/mesh_forwarder_mtd.cpp	50.00% <0.00%> (ø)
src/core/thread/mesh_forwarder.cpp	85.28% <82.60%> (-0.11%)	⬇️

... and 18 files with indirect coverage changes